DINITROGEN AND ORGANOMETALLIC CHEMISTRY OF TRIMETHYLSILYLSUBSTITUTED TRIAMIDOAMINE COMPLEXES OF MOLYBDENUM
by
MYRA BRIGID O'DONOGHUE
B.Sc. (First Class Honors)
University College Cork
(July 1988)
Submitted to the Department of Chemistry
in Partial Fulfillment of the Requirements
for the Degree of
DOCTOR OF PHILOSOPHY
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 1998
© Massachusetts Institute of Technology, 1998
i
Signature of Author
.//
C
/1
Department of Chemistry
May 19, 1998
Certified by
Richard R. Schrock
c
Accepted by
/ /
Thesis Supervisor
__
Dietmar Seyft rth
MASSACHUSETTS INSTITUTE
OF TECHNOLOGY
JUN 15 1998
LIBRARIES
Chairman, Departmental Committee on Graduate Students
This doctoral thesis has been examined by a Committee of the Department of Chemistry as
follows:
Professor Christopher C. Cummins
Chairman
Professor Richard R. Schrock
Thesis Supervisor
Professor Daniel G. Nocera
\-/.
c"------
To Mum and Dad,
for your unwavering belief in me.
Brevity is the soul of wit.
William Shakespeare
DINITROGEN AND ORGANOMETALLIC CHEMISTRY OF TRIMETHYLSILYLSUBSTITUTED TRIAMIDOAMINE COMPLEXES OF MOLYBDENUM
by
MYRA BRIGID O'DONOGHUE
Submitted to the Department of Chemistry on May 19, 1998
in Partial Fulfillment of the Requirements
for the Degree of Doctor of Philosophy in Chemistry
ABSTRACT
{[N 3 N]Mo-N=N} 2 Mg(THF)2, isolated from the reduction of [N3N]MoCl by magnesium
powder under dinitrogen, serves as an entry into the dinitrogen chemistry of molybdenum
complexes containing the TMS-TREN ligand. {[N 3 N]Mo-N=N)2Mg(THF)2 is smoothly
oxidized by ZnC12 to yield paramagnetic [N3 N]Mo(N 2 ), an X-ray study of which shows it to be a
terminal dinitrogen complex. Heating toluene solutions of [N3 N]Mo(N 2 ) under dinitrogen
affords the homobimetallic dinitrogen complex [N3 N]Mo-N=N-Mo[N 3 N]. The stepwise
reduction and functionalization of dinitrogen is achieved and examples of diazenido, hydrazido
and nitrido complexes are isolated. {[N3 N]Mo-N=N }2 Mg(THF) 2 reacts with transition metal
halides such as FeC12, VC14 (DME) and ZrC14 (THF) 2 to give heterometallic dinitrogen
complexes. The highlight of this research is the isolation of the iron-molybdenum dinitrogen
complex, {[N 3 N]Mo-N=N} 3 Fe, which contains iron in a trigonal planar coordination
environment. Magnetic susceptibility and M6ssbauer studies unequivocally demonstrate that
{[N 3 N]Mo-N=N} 3 Fe is best formulated in the solid state as a high-spin Fe(III) complex.
Addition of dimethylphosphinoethane (DMPE) to solutions of {[N3 N]Mo-N=N }3Fe yields the
tetrahedral Fe(II) complex {[N3 N]Mo-N=N} 2 Fe(DMPE). Organometallic complexes such as
[N3N]Mo(CO), [N3 N]Mo(CNtBu), [N3N]Mo(CNAr) and [N3N]Mo(C2H 4 ) are synthesized by
ligand exchange reactions employing [N3N]Mo(N2) and the appropriate ligand. [N3N]Mo(CO)
is reduced by magnesium power in the presence of TMSC1 to yield the oxycarbyne complex
[N3 N]MoCOTMS. Although [N3 N]Mo(CNAr) is thermally stable, [N3 N]Mo(CNtBu) readily
loses a tBu radical to form the cyanide complex [N3 N]MoCN. Reduction of [N3 N]MoC1 in the
absence of a donor ligand affords the cyclometallated product [bitN 3N]Mo.
Five coordinate tungsten oxo alkylidene complexes of the general type
(ArO)2W(O)(CHtBu)(PR 3 ) (Ar = 2,6-Ph 2C 6 H3 ) are synthesized by reaction of
Ta(CHtBu)(PR 3 )2 X3 (X = Cl, Br) with W(O)(OtBu) 4. 1H NMR spectroscopy reveals that only
the syn rotamer is present in solution and PPh 2 Me is labile on the NMR time scale. These
complexes are potent catalysts for the ROMP of norbornadienes, yielding polymers that are
highly cis and isotactic. The living nature of the polymerization has been demonstrated for
(ArO) 2W(O)(CHtBu)(PMe 3 )Thesis Supervisor: Professor Richard R. Schrock
TABLE OF CONTENTS
page
1
Title Page
Signature Page
Dedication
2
Quotation
4
Abstract
5
6
3
Table of Contents
List of Figures
List of Tables
List of Schemes
List of X-ray Structures
Abbreviations Used in the Text
CHAPTER 1: Derivatization of Dinitrogen in Trimethylsilyl-Substituted
Triamidoamine Complexes of Molybdenum.
INTRODUCTION
RESULTS
Activation of Dinitrogen
Synthesis of a Mo(III) Terminal Dinitrogen Complex
Synthesis of a Homobimetallic Bridging Dinitrogen Complex
Functionalization of Dinitrogen
DISCUSSION
EXPERIMENTAL PROCEDURES
REFERENCES
CHAPTER 2: Synthesis of Heterometallic Dinitrogen Complexes Containing the
{[N3 N]Mo(N2) }- Ligand.
INTRODUCTION
RESULTS AND DISCUSSION
8
10
11
12
13
16
17
22
28
34
35
55
57
65
68
69
Iron/Molybdenum Dinitrogen Complexes
Vanadium/Molybdenum Dinitrogen Complexes
71
Zirconium/Molybdenum Dinitrogen Complexes
92
97
CONCLUSIONS
86
EXPERIMENTAL PROCEDURES
REFERENCES
CHAPTER 3: Organometallic Chemistry of Trimethylsilyl-Substituted Triamidoamine
Complexes of Molybdenum.
INTRODUCTION
RESULTS
page
98
102
104
105
Synthesis of [N3 N]MO(C 2 H4)
Synthesis and Reactivity of [N3 N]Mo(CO)
Alkyl- and Arylisocyanide Complexes
108
110
Attempted Synthesis of Other Mo(III) Complexes
Synthesis and Reactivity of [bitN 3N]Mo
123
124
114
DISCUSSION
130
EXPERIMENTAL PROCEDURES
133
REFERENCES
137
CHAPTER 4: Living ROMP of Norbornadienes Employing Tungsten Oxo Alkylidene
Complexes.
INTRODUCTION
140
141
RESULTS
Synthesis of Tungsten Oxo Alkylidene Dihalide Phosphine Complexes
Synthesis of Five Coordinate Tungsten Oxo Alkylidene Complexes
Stoichiometric Olefin Metathesis Reactions of W(CHCMe 3 )(O)(O-2,6Ph 2C 6 H3 )2 (PMe 3 )
ROMP of 2,3-Disubstituted Norbornadienes Utilizing Tungsten Oxo
144
145
158
Alkylidene Catalysts
DISCUSSION
EXPERIMENTAL PROCEDURES
159
165
167
REFERENCES
174
ACKNOWLEDGMENTS
177
LIST OF FIGURES
page
CHAPTER 1
Figure 1.1.
Figure 1.2.
A view of the structure of {[N3N]Mo-N=N} 2 Mg(THF) 2.
Plot of Xm versus T for [N3N]Mo(N2).
Figure 1.3.
Figure 1.4.
A view of the structure of [N3 N]Mo(N 2 ).
Plot of Xm versus T for [N3N]Mo-N=N-Mo[N 3 N].
Figure 1.5.
Two views of the structure of {[Me-N 3 N]Mo=N-NMe 2 }OTf with
Figure 1.6.
the triflate ion omitted for clarity.
A view of the structure of {[N2 NNMe2]MoN 2 TMS }OTf with
the triflate ion omitted for clarity.
Figure 1.7.
A view of the structure of [N2 NNMe 2 ]Mo(N 2TMS)(Me).
Figure 1.8.
1H NMR
spectra of [Me-N 3 N]Mo(Me)(N 2Me 2 ) and [Me-N 3N]MoN.
CHAPTER 2
Figure 2.1.
Figure 2.2.
Structure of {[N3 N]Mo-N=N 3Fe viewed with the trigonal plane
lying in the plane of the paper.
Plot of Xm versus T for {[N3N]Mo-N=N }3Fe.
Figure 2.3.
M6ssbauer spectrum of {[N3 N]Mo-N=N} 3Fe at 77 K.
Figure 2.4.
1 H NMR
Figure 2.5.
Figure 2.6.
Mcssbauer spectrum of {[N3 N]Mo-N=N} 2 Fe(DMPE) at 77 K.
Plot of Xm versus T for {[N3 N]Mo-N=N} 3 VC1.
Figure 2.7.
1H
Figure 2.8.
Figure 2.9.
{ [N3 N]Mo-N=N} 2 VCI(THF) (lower spectrum) and 1H NMR spectrum
of a sample to which [N3N]Mo(N 2 ) was added (upper spectrum).
A view of the structure of { [N3 N]Mo-N=N} 2 VCl(THF).
A view of the structure of { [N3 N]Mo-N=N} 2 ZrCl 2.
spectra of {[N3N]Mo-N=N} 3 Fe prior to, and after addition
of THF-d8 .
NMR spectrum of a mixture of {[N3N]Mo-N=N} 3 VCl and
CHAPTER 3
Plot of Xm versus T for [N3 N]Mo(C2H 4 ).
Figure 3.1.
Plot of Xm versus T for [N3N]Mo(CO).
Figure 3.2.
89
91
94
110
113
Figure 3.3.
Plot of Xm versus T for [N3N]Mo(CNtBu).
Figure 3.4.
Plot of Xm versus T for { [N3 N]Mo(CNtBu) }OTf.
117
117
Figure 3.5.
Figure 3.6.
A view of the structure of [N3 N]MoCN.
Plot of Xm versus T for [N3N]MoCN.
119
122
page
122
Figure 3.7.
Plot of leff versus T for [N3 N]MoCN.
Figure 3.8.
Figure 3.9.
Two views of the structure of [bitN 3 N]Mo.
Plot of Xm versus T for [bitN3N]Mo.
125
128
Four possible regular structures of 2,3-disubstituted norbornadienes.
A view of the structure of W(CHCMe 2 Ph)(OtBu) 2Br 2 .
Variable temperature 1H NMR spectra of
142
148
CHAPTER 4
Figure 4.1.
Figure 4.2.
Figure 4.3.
Figure 4.5.
(DPPO) 2W(O)(CHCMe 3)(PPh 2 Me).
150
1
Variable temperature H NMR spectra of (DPPO)2 W(O)(CHCMe 3 )(PPh 2 Me)
illustrating exchange between free and bound PPh2Me.
152
A view of the structure of (DPPO)2 W(CHCMe 3)(0)(PPh 2 Me).
154
Figure 4.6.
Variable temperature 1H NMR spectra of
Figure 4.4.
(DPPO) 2 W(O)(CHCMe 2 Ph)(PPh 2 Me).
Figure 4.7.
Figure 4.8.
155
Number Average Molecular Weight (Mn) of Poly(DCMNBD) versus
Equivalents of DCMNBD added to (DPPO) 2 W(O)(CHCMe 3 )(PMe 3 )
in CH 2 CI 2.
13 C NMR
spectrum of poly(DCMNBD) produced using
163
(DPPO) 2 W(O)(CHCMe 3 )(PMe 3 ).
164
LIST OF TABLES
page
CHAPTER 1
Table 1.1.
IR and
15 N
NMR data for selected complexes.
Crystallographic data, collection parameters and refinement
23
parameters for {[N3 N]Mo(N 2 )12 Mg(THF) 2 and [N3 N]Mo(N 2 ).
Selected bond lengths and bond angles for {[N3N]Mo(N 2 )) 2 Mg(THF) 2 .
Selected metrical parameters for crystallographically
26
characterized complexes.
28
33
Table 1.7.
Table 1.8.
Selected bond lengths and bond angles for [N3 N]Mo(N 2 ).
Crystallographic data, collection parameters and refinement parameters for
{[Me-N 3 N]MoN 2 Me 2 }OTf and {[N2NNMe 2 ]MoN 2TMS }OTf.
Selected bond lengths and bond angles for {[Me-N 3N]MoN 2 Me2 }OTf.
Selected bond lengths and bond angles for {[N2 NNMe 2 ]MoN 2 TMS) OTf.
Table 1.9.
Selected bond lengths, bond angles and dihedral angles for
47
Table 1.10.
[N 2 NNMe 2 ]Mo(N 2 TMS)(Me).
Crystallographic data, collection parameters and refinement parameters for
[N2 NNMe 2 ]Mo(N 2TMS)(Me).
48
Table 1.2.
Table 1.3.
Table 1.4.
Table 1.5.
Table 1.6.
27
39
40
43
CHAPTER 2
Table 2.1.
Crystallographic data, collection parameters and refinement parameters for
{[N3N]Mo-N=N} 3Fe and {[N3 N]Mo-N=N} 2 VCI(THF).
Table 2.2.
Table 2.3.
Selected bond lengths and bond angles for {[N3 N]Mo-N=N} 3 Fe.
Selected metrical parameters for heterometallic dinitrogen complexes.
Selected bond lengths and bond angles for {[N3 N]Mo-N=N} 2 VCI(THF).
Crystallographic data, collection parameters and refinement parameters for
{[N3N]Mo-N=N }2 ZrCl 2 .
Selected bond lengths and bond angles for {[N3 N]Mo-N=N} 2 ZrCl 2 .
Table 2.4.
Table 2.5.
Table 2.6.
CHAPTER 3
Table 3.1.
Table 3.2.
Table 3.3.
Table 3.4.
Crystallographic data, collection parameters and refinement parameters for
[N 3 N]MoCN and [bitN 3 N]Mo.
Selected bond lengths and bond angles for [N3 N]MoCN.
Selected bond lengths and bond angles for [bitN 3 N]Mo.
Selected characterization data for paramagnetic [N3 N]Mo complexes.
120
121
126
128
page
CHAPTER 4
Crystallographic data, collection parameters and refinement
Table 4.1.
parameters for W(CHCMe 2 Ph)Br 2 (OtBu) 2 and
W(CHCMe 3 )(O)(O-2,6-Ph2C 6 H3)2(PPh2Me).
Table 4.2.
Table 4.3.
Table 4.4.
Table 4.5.
Table 4.6.
Table 4.7.
146
Selected bond lengths and bond angles for W(CHCMe2Ph)Br 2 (OtBu)2.
NMR Data for five coordinate tungsten oxo alkylidene complexes.
Selected bond lengths and bond angles for
147
147
W(CHCMe 3 )(0)(O-2,6-Ph2C 6 H3 )2 (PPh2 Me).
154
GPC characterization of all cis, isotactic poly(DCMNBD) prepared using
W(CHCMe 3 )(O)(O-2,6-Ph 2 C6 H3 )2 (PPh2 Me).
160
GPC characterization of all cis, isotactic poly(DCMNBD) prepared using
W(CHCMe 2 Ph)(O)(O-2,6-Ph 2C 6 H3 )2 (PPh 2 Me).
GPC characterization of all cis, isotactic poly(DCMNBD) prepared using
161
W(CHCMe 3 )(O)(O-2,6-Ph2C 6 H3 )2(PMe 3 ).
162
LIST OF SCHEMES
CHAPTER 1
Scheme 1.1. Derivatization of dinitrogen in [N3N]Mo complexes.
Scheme 1.2. Proposed mechanism for the formation of {[Me-N 3N]Mo=N-NMe 2 ) OTf
and {[N2 NNMe 2 ]MoN 2 TMS }OTf.
CHAPTER 2
Scheme 2.1. Synthesis of heterometallic dinitrogen complexes.
Scheme 2.2. Possible mechanisms for the formation of {[N3N]Mo(N 2 ) }3Fe
CHAPTER 3
Scheme 3.1.
Organometallic chemistry of [N3 N]Mo complexes.
107
LIST OF X-RAY STRUCTURES
IDENTIFICATION NUMBER
page
CHAPTER 1
{[N3 N]Mo(N 2 )12 Mg(THF)2
96169
25
[N3N]Mo(N 2 )
{[Me-N 3N]Mo=N-NMe2 }OTf
{[N2 NNMe 2 ]MoN 2TMS }OTf
96170
32
97023
97062
38
42
[N2 NNMe 2 )]Mo(N 2 TMS)(Me)
97071
49
96166
96203
97096
75
[N3 N]MoCN
97155
119
[bitN 3 N]Mo
96150
125
95083
148
95038
154
CHAPTER 2
{[N3N]Mo(N 2 )13 Fe
{[N3N]Mo(N 2 ) }2 VC1(THF)
{[N3N]Mo(N 2 )12 ZrC12
91
94
CHAPTER 3
CHAPTER 4
W(CHCMe 2 Ph)(OtBu) 2 Br 2
W(CHtBu)(O)(O-2,6-Ph 2C 6 H3 )2 (PPh2 Me)
ABBREVIATIONS USED IN THE TEXT
Ar
ax
aryl
axial
br
calcd
broad
Ca
carbon bound to metal
CP
carbon bound to Ca
carbon bound to Nax
carbon in the ipso position of an aromatic ring
CP, ax
Cipso
Cp
Cp'
Cp*
15-crown-5
d
DCMNBD
DME
calculated
C5 H5
C5 H4 Me
C5 Me 5
1, 4, 7, 10, 13-pentaoxacyclopentadecane
doublet
2,3-dicarbomethoxynorbornadiene
1,2-dimethoxyethane
DMPE
DPPO
dimethylphosphinoethane
diphenylphenoxide
Bu
tBu
butyl
eq
Et
equatorial
[Et 3 Si-N 3 N]
eV
FcOTF
GPC
h
tertiary butyl
ethyl
[(Et 3 SiNCH 2 CH 2 )3N]3electron volts
ferrocenium triflate
gel permeation chromatography
hours
Ha
Hz
hydrogen (proton) bound to Ca
Hertz
IR
Infrared
nJAB
m
A-B coupling constant through n bonds
multiplet
Me
methyl
[Me-N 3 N] 3min
[N(CH 2 CH 2 NTMS) 2 (CH 2 CH 2 NMe)] 3 minutes
[N 3 N]3-
number average molecular weight
weight average molecular weight
[N(CH 2 CH 2 NTMS) 2 (CH 2 CH 2 NMe 2 )]2 [(Me 3 SiNCH 2 CH 2 )3 N] 3 -
[N3NF] 3 -
[(C6 F 5NCH 2 CH 2 ) 3N] 3-
Na
nitrogen bound to metal
Np
nitrogen bound to Na
na
not available
NBDF6
NMR
2,3-bis(trifluoromethyl)norbornadiene
nuclear magnetic resonance
Np
OTf
neopentyl
Mn
Mw
[N 2 NNMe 2 ] 2 -
PDI
Ph
ppm
Pr
iPr
py
q
ROMP
O3 SCF 3 , triflate, trifluoromethanesulfonate
polydispersity index (Mw/Mn)
phenyl
parts per million
propyl
isopropyl
pyridine
s
quartet
ring-opening metathesis polymerization
room temperature
singlet
SQUID
superconducting quantum interference device
t
triplet
THF
tol
TMS
TMS-TREN
TREN
UV
VT
tetrahydrofuran
toluene
r.t.
Xm
trimethylsilyl, Me 3 Si
[(Me 3 SiNCH 2 CH 2 )3 N]32, 2', 2" - triaminotriethylamine, N(CH 2 CH 2 NH2 )3
ultraviolet
variable temperature
molar magnetic susceptibility
chemical shift in parts per million
AV1/2
peak width at half-height
kmax
extinction coefficient at wavelength of maximum optical absorption
wavelength of maximum optical absorption
v
frequency
9
gB
magnetic moment
Bohr magneton
geff
effective magnetic moment
CHAPTER 1
Derivatization of Dinitrogen in Trimethylsilyl-Substituted
Triamidoamine Complexes of Molybdenum
A portion of the material covered in this chapter has appeared in print:
M~sch-Zanetti, N. C., Schrock, R. R., Davis, W. M., Wanninger, K., Seidel, S. W.,
O'Donoghue M. B. J. Am. Chem. Soc. 1997, 119, 11037.
O'Donoghue, M. B., Zanetti, N. C., Davis, W. M., Schrock, R. R. J. Am. Chem. Soc.
1997, 119, 2753.
ChapterI
INTRODUCTION
Dinitrogen is the most abundant component of the Earth's atmosphere and is chemically
rather inert. N2 has a high bond dissociation energy (225 kcal/mol), high ionization potential
(15.058 eV) and negative electron affinity (-1.8 eV). 1 The industrial synthesis of ammonia from
its elements is achieved by the Haber-Bosch process in which dinitrogen is reduced at high
temperatures and pressures in the presence of an iron catalyst, and millions of tons of ammonia are
produced in this manner every year (equation 1). In contrast, nitrogenase enzymes found in
bacteria in the roots of legumous plants achieve the same conversion at ambient temperatures and
pressures (equation 2).
N2 + 3 H2
N2 + 8 H + 8 e
350-650 oC, 200-400 atm
Fe catalyst
ntrogenase
2 NH3
2 NH 3 + H 2
(1)
(2)
Three types of nitrogenases are now known, containing Fe/Mo, V/Fe and Fe centers. 2' 3
The crystal structure of the FeMo cofactor of nitrogenase isolated from Azotobacter vinelandii has
been refined to 2.2 A resolution and provides tantalizing clues as to how dinitrogen might be
activated and reduced in biological systems. 4 Salient features include the presence of two cubane
fragments, Fe4 S3 and Fe3MoS 3 , bridged by inorganic sulfurs, molybdenum in an octahedral
environment, and six trigonally-coordinated iron atoms. It is not immediately obvious from the
structure if or how dinitrogen could be activated at the apparently coordinatively-saturated
molybdenum center, but it must be noted that the enzyme is in a resting state and the exact
mechanism by which dinitrogen is bound and reduced by nitrogenase is unknown.
A common thread linking the industrial synthesis of ammonia and biological N2 fixation is
the presence of transition metals. The challenge to the inorganic chemist has been the reduction
and functionalization of dinitrogen to ammonia utilizing well-defined transition metal complexes.
References begin on page 65
Chapter1
Work in this area was initiated by the discovery of the first dinitrogen complex [Ru(NH 3 )5 (N2 )+
by Allen and Senoff in 1965. 5 Ironically, the dinitrogen ligand in this complex is derived from
hydrazine and not free dinitrogen. Nevertheless, isolation of this complex provided the first
unequivocal evidence for the activation of dinitrogen by discrete transition metal complexes. In the
following three decades numerous other transition metal dinitrogen complexes have been isolated
and characterized and examples of stable N2 complexes exist for all metals from Group 4 through
to Group 10 with the exception of palladium and platinum. 6
Extensive work on the
functionalization of dinitrogen in low oxidation state complexes of type M(N 2 )2L 4 (M = Mo, W; L
= phosphine) has been carried out by the groups of Chatt, Leigh and Hidai and several
comprehensive reviews of this chemistry have appeared. 6 -8 Protonation of these complexes by
strong acids leads to the isolation of diazenido, hydrazido and hydrazidium complexes and in the
presence of excess acid ammonia is produced. C-N bond formation has been extensively studied
in M(N 2 )2 (P-P) 2 complexes (M = Mo, W; P-P = chelating diphosphine) and the synthesis of
organonitrogen compounds such as pyrrole and pyridine has been demonstrated. 9 However, in
general the fate of the metal-containing species has not be determined. Earlier work in the Schrock
group centered on high oxidation complexes of Mo and W containing the Cp*MMe3 core and the
catalytic reduction of hydrazine to ammonia has been documented in these systems. 10- 12 More
recently, two unprecedented reactions of dinitrogen have been discovered; Cummins has shown
that homolytic cleavage of the N-N triple bond can be achieved by the three-coordinate
molybdenum complex Mo[N(R)Ar]3 (R = C(CD 3 )2 CH 3 , Ar = 3,5-C6H 3 Me2) to yield
NMo[N(R)Ar] 3 13'14 and Fryzuk has observed the first reaction of dihydrogen with bound
dinitrogen. 15
Current work in our group has focused on the synthesis of transition metal complexes
containing triamidoamine ligands and an evaluation of their chemistry in the context of dinitrogen
reduction. In particular we are interested in exploring the chemistry of complexes of the type
MN2Rx (x = 0-4) and MNR (x = 0-3) as well as multimetallic dinitrogen complexes so as to
delineate what factors are of fundamental importance to the activation and reduction of dinitrogen.
References begin on page 65
Chapter1
Ligands of the type [(RNCH 2 CH 2 )3N] 3 - can bind to a variety of main group elements 16'
17
and
transition metals in oxidation state 3+ or higher. When R is a sterically bulky group such as
trimethylsilyl, opportunities to study rarely observed complexes and decomposition pathways have
arisen. Examples include preparation of a tantalum phosphinidene complex, 18 preparation of
molybdenum and tungsten phosphido and arsenido complexes, 19 and a demonstration that certain
molybdenum and tungsten alkyl complexes decompose via a elimination as much as six orders of
20 2 1
magnitude faster than via 3 elimination. ,
Triamidoamine ligands usually bind to transition metals in a tetradentate manner thus
creating a sterically-protected, three-fold symmetric pocket in which to bind small molecules. We
have been interested in exploiting the sterically-protected apical site and the orbital arrangement in
this pocket to bind and activate dinitrogen. The orbitals available to bind ligands such as dinitrogen
consist of a a orbital (approximately dz2) and two degenerate it orbitals (approximately dxz and
dyz). When these orbitals are compared with those that dinitrogen utilizes to bind "end-on" to a
metal center, namely, the orbital containing the lone pair and the pair of degenerate tn*orbitals, it
appears that triamidoamine complexes are well-suited to bind dinitrogen, assuming the metal and
dinitrogen orbitals are matched in terms of energy. This appears to be the case and we recently
showed that [N3NF]MoCl could be reduced with sodium under dinitrogen by two electrons to give
the sodium "diazenido" complex, [N3NF]Mo-N=N-Na(ether)x, and by one electron to give the
homobimetallic complex, [N3NF]Mo-N=N-Mo[N3NF] ([N3NF] 3- = [(C6 F5 NCH 2 CH 2 )3 N]3-). 22
Furthermore, it proved feasible to isolate a neutral Re(lI) complex, [N3NF]Re(N2). 23
Although much is unknown about the mechanism of the binding and reduction of N2 in
biological systems, it has been long recognized that transition metals such as molybdenum are
essential elements for activity. It is also accepted that the coordination of dinitrogen to a metal
center is a prerequisite for reduction and further transformation. A survey of the literature suggests
that in seeking to prepare well-defined transition metal dinitrogen complexes, the selection of
reductant and reducible precursor is all-important. However, the subtleties that govern such
choices are difficult to rationalize and studies of new systems require the investigator to explore all
References begin on page 65
Chapter1
avenues in search of a potential entry into dinitrogen chemistry. Prior to this work a syntheticallyuseful entry into the dinitrogen chemistry of silylated triamidoamine complexes was lacking.
{[(tBuMe 2 SiNCH 2 CH 2 )3N]Mo }2(g-N2) was the singular example of a dinitrogen complex in
these systems, having been isolated in <10% yield from the reaction of MoC13 (THF) 3 with
Li3 N3N. 24 Spurred on by the results obtained with [N3NF]Mo complexes, we initiated a study of
the related [N3 N]Mo complexes and the chemistry described in this chapter charts our progress
toward the derivatization of dinitrogen at a single metal center culminating with N-N bond cleavage
and is summarized in Scheme 1.1. With the exception of two, all of the complexes reported are
diamagnetic and so are easily characterized by standard spectroscopic methods. Five X-ray
crystallography structures are reported including the first example of a hydrazido complex in
TREN-based systems.
In particular, X-ray crystallography proved to be a useful tool in
delineating the course of reactions in which the TMS-TREN ligand has become involved in the
chemistry. The susceptibility of the [N3 N] 3- ligand toward degradation via Si-N bond cleavage
has been documented by the isolation of several complexes in which a TMS group has been
replaced by one or more methyl groups. Although this type of reactivity was not fully anticipated
and in general is undesired, it has given rise to new types of diamido/bisdonor complexes that may
be of use in future research.
References begin on page 65
Chapter1
Scheme 1.1. Derivatization of dinitrogen in [N3 N]Mo complexes.
N
TMS
N
TMS
II
TMS
TMS
N
TMS
"M
TMS*#N "I"Mo-N
S
TM
I
..
/MS
A
III
N
TMS
N
4
IN
TMS
ZnC12
TMS
N
TMS
N
TMS
/TMs
13,14
TMSCI
MS.JTMsN
TMs 9 "N Mo -N/
-.
-M
TMs ""I
Mo-N
{[N 3N]Mo(N 2)
ROTf
2 Mg(THF)2
1
aN
T"MS
Mg, N2
.'
CH30Tf
TMS
Cl
S
I
N
TMS
TMSN "" Mo -N
H, 3
/CH
(wM!-
3
N
TMS
TMSN:
TM,
N
IICH
3
N
/CH3
TMS
+ {[N2NNMe 2]MoN 2TMS)}+
Mo-N
8
%~ia..Mo-
A
H3 SC /CH3
CH 3MgC1
TMS_
TMS
N
N
Mo
N NJ
References begin on page 65
CH3
11
+ CH 4 + (CH3)2 NH
ChapterI
RESULTS
Activation of Dinitrogen
A solution of [N3 N]MoCl in THF is reduced cleanly by magnesium powder under
dinitrogen to give a mixture of two diamagnetic products in a ratio of 1:3, according to their
respective TMS resonances in 1H NMR spectra. Efforts to separate these products via fractional
crystallization were unsuccessful. However, addition of 1,4-dioxane to a toluene solution of the
mixture allows one of the products to be isolated in high yield (90%) as an orange crystalline solid.
The 1H NMR spectrum of this product, {[N3N]Mo-N=N} 2 Mg(THF) 2 (1; equation 3), consists of
a single TMS resonance and a pair of triplets for the methylene protons on the ligand backbone
characteristic of compounds in which the [N3 N]Mo portion of the molecule is C3-symmetric. The
1H
NMR spectrum also shows there to be one molecule of THF present per [N 3 N]Mo unit. If the
Mg, THF, N2
2 [N3N]MoC1
1,4-dioxane
{[N3N]Mo-N=N}2Mg(THF) 2 + MgCl 2 (dioxane)
1
1
reduction of [N3 N]MoCl is carried out under
15N
2,
the
15 N
(3)
NMR spectrum of the product in C6 D6
consists of a pair of doublets at 377.0 and 304.4 ppm (JNN= 12 Hz) corresponding to Na and Np
of 1- 15 N2 respectively. 25 The IR spectrum of 1 has a strong broad stretch at 1719 cm -1 that
shifts to 1662 cnrm
in 1- 15 N2 . IR and
15 N
NMR data for selected complexes reported in this
chapter are summarized in Table 1.1. The second diamagnetic product present in approximately
25% yield before the addition of dioxane is proposed to be {[N3 N]Mo-N=N}MgCI(THF) 2 (la),
and the yield of 1 is believed to be raised to 90% (isolated) as a consequence of a Schlenk-like
equilibrium that yields MgCl2(dioxane) (equation 4). Further support for this suggestion comes
from the observation that addition of TMSC1 to a mixture of 1 and la yields [N3 N]MoN 2TMS
(see below) as the sole product in high yield.
2 { [N3 N]Mo-N=N}MgCl(THF) 2
la
References begin on page 65
1,4-dioxane
1 + MgCl 2 (dioxane)
(4)
Chapter 1
Table 1.1. IR and
15 N
NMR data for selected complexes.
Complex
V14N14N
V15N15N
6 Na
SN5
1
1719 a
1662 a
377.0b
304.4
4
1934c
1870c
6
1714 a
1654 a
356.9 b
238.1
7
374.8a
157.2
8
361.5a
244.3
374.6b
239.5
354.9 a
142.0
9
1640d
1577 d
10
1002d
11
866. lb
9 77 d
ain THF, bin C6 D6 , in pentane, din Nujol.
One equivalent of magnesium powder is required for complete reduction of [N3 N]MoCl
but the reaction is unaffected by the presence of excess magnesium powder. The reaction is
solvent dependent and reduction only occurs in the presence of THF. The sodium analog of 1,
{ [N3 N]Mo-N=N) [Na(15-crown-5)] (2) is accessed via reduction of [N3 N]MoCl with two
equivalents of sodium naphthalenide followed by addition of one equivalent of 15-crown-5.
Orange, diamagnetic 2 is isolated in 65% yield and the IR spectrum of 2 in Nujol exhibits a strong
N-N stretch at 1791 cm-1 . For comparison, the IR spectrum of the related complex {[N3NF]MoN=N} [Na(15-crown-5)] has an N-N stretch at 1848 cm-1. 2 2 It should be noted that the choice of
magnesium as a reductant has several advantages over sodium naphthalenide in that it is readily
available, does not require preactivation and can easily be separated from the desired product.
[N3 N]MoOTf 2 1 is not a viable starting material for entry into dinitrogen chemistry; reduction of
[N3N]MoOTf
by
magnesium
yields
the
known
dimeric
complex,
{(TMSNCH 2 CH 2 )2 N(CH 2 CH 2 N)Mo 1226 (3) via formal loss of TMSOTf (equation 5). This
References begin on page 65
Chapter1
reaction is illustrative of the importance of one's choice of precursor complex in gaining access to
the dinitrogen chemistry of [N3 N]Mo complexes.
TMS
2 [N3N]MoOTf + xs Mg
THF, N2
- 2 TMSOTf
TMS,
N=Mo
.T
-MS
N
Mo
mN
3
(5)
A toluene-d8 solution of 1 shows no signs of decomposition upon being heated to 82 'C
under dinitrogen for 24 h. Furthermore, a solution of 1 stored at room temperature under
dinitrogen remains unchanged over a period of two weeks (according to 1H NMR spectroscopy).
However, 1 apparently decomposes rapidly in the solid state when exposed to high vacuum as
evidenced by a color change from bright orange to dark brown. We speculate that loss of THF is
the first step in this decomposition although no products of the reaction have been identified.
Crystals of 1 suitable for an X-ray study were grown from saturated diethyl ether solutions
at -20 *C; a half a molecule of diethyl ether was found in the unit cell. Crystallographic data and
collection and refinement parameters are given in Table 1.2. The molecular structure of 1 along
with the atom-labeling scheme is shown in Figure 1.1, while pertinent bond lengths and bond
angles are listed in Table 1.3. Table 1.4 summarizes selected metrical parameters for all of the
crystallographically-characterized complexes reported in this chapter. 1 is comprised of two
{[N3 N]Mo(N 2 )}- units bound to pseudo-tetrahedral magnesium, the coordination sphere being
completed by two molecules of THF. The N-Mg-N bond angle opens to 134.70 in order to
References begin on page 65
i
--
--
I
--
-
-~
--
Chapter1
accommodate the sterically bulky {[N3 N]Mo(N 2 ) }- "ligands"; the Mo-N-N-Mg linkages are
essentially linear. The N-N bond lengths (1.164(13) and 1.195(13)
A)
are indicative of some
reduction of the dinitrogen ligands in 1 compared with free dinitrogen (1.098
A2 7 )
and are
consistent with formulation of 1 as a diazenido complex of Mo(IV). We have found that the
twisting of a given TMS group out of the Nax-M-Neq plane and the resulting decrease in the NaxM-Neq-Si dihedral angle are useful measures of the degree of steric strain in the pocket in [N3 N]
complexes. For example, in [N3N]Mo(cyclohexyl) 2 1 this angle was found to range from 1290 to
1360 as a consequence of the steric interaction between the cyclohexyl ring and the TMS groups of
the ligand. In the case of 1, this angle is found to average to 173.70, consistent with there being
little steric pressure in the pocket. Examples of Mg 2 + salts of diazenido complexes have been
crystallographically-characterized including {(PMe3) 3 Co(N 2 )
2 Mg(THF) 4 28
which contains
magnesium in a pseudo-octahedral environment.
Figure 1.1. A view of the structure of {[N3N]Mo-N=N) }2 Mg(THF) 2 (1).
0(1
Si(6)
0(2)
N(23)
N(101)
N(202)
Mo(2)
N(24)
References begin on page 65
N(11)
Mo(l)
N(102)
N(201)
N(12
N(14)
Chapter1
Table 1.2. Crystallographic data, collection parameters and refinement parameters for 1 and 4.
4
Empirical Formula
C 4 0 H 9 8MgMo 2N 12 0 2 .50Si 6
C 15 H39MoN 6 Si 3
Formula Weight
1172.03
483.73
Diffractometer
SMART/CCD
SMART/CCD
Crystal Dimensions (mm)
0.33 x 0.26 x 0.20
0.46 x 0.12 x 0.12
Crystal System
Triclinic
Orthorhombic
Space Group
Pi
Pbca
a(A)
10.1540(2)
17.0164(2)
b (A)
16.4300(3)
16.9922(3)
c(A)
19.8388(5)
34.251
a( 0)
89.4350(10)
90
a (0)
84.1230(10)
90
Y(o)
82.19
90
V (A3), Z
3261.77(12), 2
9903.7(2), 16
Deale (Mg/m3)
1.193
1.298
Absorption coefficient (mm-l)
0.543
0.686
Fooo
1244
4080
Temperature (K)
183(2)
183(2)
O range for data collection (0)
1.25 to 20.00
1.19 to 23.27
Reflections collected
9883
30723
Unique Reflections
6054
7087
R
0.0905
0.0459
Rw
0.1016
0.0554
GoF
1.162
1.245
References begin on page 65
Chapter1
Table 1.3. Selected bond lengths and bond angles for { [N3 N]Mo-N=N} 2 Mg(THF)2 (1).
Bond Lengths (A)
Mo(1)-N(101)
1.876(11)
Mo(2)-N(201)
1.842(10)
N(101)-N(102)
1.164(13)
N(201)-N(202)
1.193(13)
Mg-N(202)
1.966(11)
Mo(1)-N(14) 2.215(10)
Mo(2)-N(24) 2.252(9)
Mo(1)-N(l11)
1.998(12)
Mo(1)-N(12) 2.001(11)
Mo(1)-N(13) 2.010(11)
Mo(2)-N(21) 1.979(10)
Mo(2)-N(23) 2.027(10)
Mg-O(1)
Mg-N(102)
1.973(11)
2.041(10)
Mo(2)-N(22) 2.017(9)
Mg-O(2)
2.019(10)
Bond Angles (deg)
Mo(1)-N(101)-N(102)
175.6(9)
Mo(2)-N(201)-N(202)
177.0(9)
Mg-N(102)-N(101)
178.2(9)
Mg-N(202)-N(201)
166.6(9)
Mo(1)-N(ll)-Si(1)
127.7(6)
Mo(2)-N(23)-Si(6)
126.0(6)
N(102)-Mg-N(202)
134.7(5)
O(1)-Mg-O(2)
N(102)-Mg-O(1)
107.9(4)
N(102)-Mg-O(2)
104.5(4)
N(202)-Mg-O(1)
105.1(4)
N(202)-Mg-O(2)
102.8(4)
94.7(5)
Dihedral Angles (deg)a
N(14)-Mo(1)-N( 11)-Si(1)
179.45
N(14)-Mo(1)-N(13)-Si(3)
-180.00
N(24)-Mo(2)-N(21)-Si(5)
167.36
N(24)-Mo(2)-N(22)-Si(4)
-171.08
aObtained from a Chem-3D Drawing
References begin on page 65
ChapterI
Table 1.4. Selected metrical parameters for crystallographically characterized complexes.
Complex
N-N (A)
Mo-N (A)
N-N-R (deg)
1
1.164(13)
1.876(11)
178.2(9)
1.193(13)
1.842(10)
166.6(9)
1.085(5)
1.990(4)
1.083(6)
1.995(4)
1.334(13)
1.747(10)
4
7
119.1(10)
115.9(1)
8
1.206(9)
1.803(7)
170.5(8)
9
1.229(3)
1.789(2)
137.0(2)
Synthesis of a Mo(III) Terminal Dinitrogen Complex
Since 1 and 2 arise from the two electron reduction of [N3 N]MoCl and the activation of
dinitrogen, we wondered if the neutral terminal dinitrogen complex [N3 N]Mo(N 2 ) (4) would be
isolable. Although paramagnetic 4 can be isolated from the reduction of [N3 N]MoCl in THF by
sodium naphthalenide, the reaction is neither clean nor reproducible. The reaction is sensitive to a
number of factors that include temperature, efficiency of stirring, and rate of addition of the
reductant. For example, if one equivalent of sodium naphthalenide is added dropwise to a THF
solution of [N3 N]MoCI followed by addition of TMSC1, the 1 H NMR spectrum of the crude
reaction mixture shows 0.5 equivalents of unreacted [N3 N]MoCl and 0.5 equivalents of
[N 3 N]MoN2 TMS (see below) to be present, indicating that two electron reduction of [N3N]MoCl
has occurred exclusively. If one equivalent of sodium naphthalenide is added to [N3 N]MoCl in
THF all at once, the main product of the reaction is 4, but it is contaminated with [N3N]MoC1. In
contrast, the one electron reduction of [N3NF]MoOTf by sodium amalgam leads to the
homobimetallic dinitrogen complex, [N3NF]Mo-N=N-Mo[N3NF] and not the terminal dinitrogen
complex. 22
References begin on page 65
Chapter1
As 4 is formally the one electron oxidation product of {[N3N]Mo(N2) }-, we reasoned that
it might be accessible via oxidation of 1. 1 reacts with MC12 (PPh 3 )2 (M= Pd, Ni) according to
equation 6 and 4 can be isolated as burgundy colored crystals from this reaction in >80% yield
although occasionally samples are contaminated with 5-7% [N3N]MoC1. To circumvent this
problem other oxidants were sought and a cleaner, high-yield route to 4 results from the reaction of
THF
{ [N3N]Mo-N=N }2 Mg(THF) 2 + ML 2C12
F --
-20 oC
1
MgC12 + [N3N]Mo(N 2 ) + MLx
(6)
4
M = Pd, Ni; x is unknown
1 with ZnC12 (equation 7). Use of ZnC12 as the oxidant simplifies workup as metallic zinc
precipitates out of solution during the course of the reaction and 4 is easily separated from MgC12
by extraction into pentane. Also, 4 is isolated free of any contamination by [N3N]MoC1. It should
be noted that oxidation of [N3NF]Mo(N2){Na(ether)x} by ferrocenium triflate yields the
homobimetallic dinitrogen complex, [N3NF]Mo-N=N-Mo[N3NF] and not the terminal dinitrogen
complex [N3NF]Mo(N2). 22 This result suggests that the extent of backbonding into the 7t* orbitals
of dinitrogen is greater in [N3N]Mo complexes compared to [N3NF]Mo complexes, allowing
isolation of 4. Such a suggestion is sensible in view of the fact that the [N3N] 3- ligand is more
electron-donating then the [N3NF] 3- ligand, giving rise to more electron-rich metal centers in
[N3N]Mo complexes compared to [N3NF]Mo complexes.
N
TMSMS
1 + ZnC12
THF, -20 OC
TMS
f'"
1N
III
Mo-
+ Zn + MgCl 2
4
(7)
References begin on page 65
Chapter1
The 1H NMR spectrum of 4 consists of two broad resonances at 14.02 and -40.57 ppm for
the methylene protons of the ligand and a sharper resonance at -4.53 ppm assigned to the TMS
groups of the ligand. Such a spectrum, that is one exhibiting a high field and a low field resonance
for the ligand methylene protons is also characteristic of [N3N]WIII(L) 29 and [N3NF]WIII(L) 30
complexes. The IR spectrum of 4 in pentane has a strong absorption at 1934 cm-1 that shifts to
1870 cm - in 4- 15 N 2 (Table 1.1).
The IR spectrum of 4 in Nujol consists of two strong
absorptions at 1910 and 1901 cm-l. We attribute the two absorptions in the solid state spectrum to
the presence of two molecules in the unit cell (see below) and IR spectra of [N3 N]MoCO exhibit
similar features (see Chapter 3).
SQUID3 1 magnetic susceptibility data for solid 4 is plotted versus temperature in Figure
1.2. Fitting these data to the Curie law over the temperature range 5-300 K yields t = 1.75(1) gB.
A trigonal bipyramidal complex possessing C3v symmetry has a ligand field splitting pattern in
which the lowest lying orbitals are the degenerate dxz/dyz pair. In the case of 4, a Mo(III)
complex, three electrons occupy these two orbitals giving rise to a single unpaired spin, as
evidenced by the magnetic susceptibility data. Although 4 is stable under dinitrogen, dinitrogen
exchange does take place slowly. If a toluene solution of 4 is stirred under an atmosphere of 15 N2
for one week, the IR spectrum of the resulting solid in Nujol reveals four strong absorptions at
1910, 1901, 1846 and 1839 cm- 1 consistent with exchange of 14 N2 with
15 N
2
to yield a roughly
2:1 mixture of 4- 14 N2 and 4- 15 N2. In contrast, the dinitrogen ligand in [N3NF]Re(N2), 23 the
only other example of a terminal dinitrogen complex in TREN-based systems, is not labile and
consequently exchange with
15 N
2
is not observed. We have found that the lability of N2 in 4 can
be exploited to isolate other [N3 N]MoIII(L) complexes and details of this chemistry are the subject
of Chapter 3. 4 reacts with TMSOTf to give a mixture of [N3 N]MoOTf 2 1 and [N3 N]MoN 2 TMS
(6) according to 1 H NMR spectroscopy. 4 is cleanly reduced by magnesium powder in THF to
give 1 in high yield with no trace of the closely related species proposed to be la that is formed
upon reduction of [N3 N]MoCl by Mg in THF (see above). The electrochemical behavior of 4
mirrors its chemical behavior. The cyclic voltammogram of 4 (obtained by Dr. Luis Baraldo of the
References begin on page 65
Chapter1
Cummins group) indicates that 4 is reduced at -1.9 eV (versus ferrocene/ferrocenium) to
{[N3N]Mo(N 2)}- and that the reduction is reversible. Oxidation of 4 is irreversible presumably
due to loss of dinitrogen from the cationic d2 metal center as a consequence of decreased
backbonding into the i7* orbitals of dinitrogen.
Figure 1.2. Plot of Xm (corrected for diamagnetism using Pascal's constants) versus T for
[N3N]Mo(N 2), (4).
...............
...............................
..................
:.................
--.................
.................
0.08
0.07
0.06
0.05
X
0.04
0
0.03
0............
....................................................
.
.......
.................
..................
0.02
0
0.01
..............
...... .o
..........
.........
...................
...........
......
...........
..................
•
.......... .......... ............................
0
0
50
100
150
200
T (K)
250
300
350
Single crystals of 4 were grown from saturated diethyl ether solutions at -20 'C and
examined in an X-ray study. Crystallographic data and collection and refinement parameters are
given in Table 1.2. The molecular structure of 4 along with the atom-labeling scheme is shown in
Figure 1.3 while selected bond lengths and bond angles are listed in Table 1.5. Two statisticallyidentical molecules were found in the unit cell. The molybdenum atom is displaced from the plane
defined by the amide nitrogens by 0.304 A in the direction of Na. 4 contains an "end-on"
dinitrogen ligand with a linear Mo-N-N linkage. The N-N bond length at 1.085(5) A is not
References begin on page 65
iile
-
--c------
-
-- '-1-
-- --
---^ -
-------- --
Chapter1
statistically different from that of free dinitrogen (1.098
A27 ), which
suggests that there is little
reduction of the dinitrogen ligand in 4. The short N-N bond length and long Mo-N bond length
(1.990
A)
are consistent with the observed lability of dinitrogen. In 4 each MoN 2 C 2 five-
membered ring has an envelope conformation with CI, ax serving as the 'flap' of the envelope.
Consequently, the TMS groups of 4 are all oriented upright, the Nax-M-Neq-Si dihedral angles
averaging to 175.60.
Figure 1.3. A view of the structure of [N3 N]Mo(N 2 ) (4).
N(6A)
Q
Si(1A)
cor
The isolation and structural characterization of 4 demonstrates, for the first time, the ability
of a d3 Mo center in a triamidoamine complex to bind dinitrogen, a question that work with
[N3NF]Mo complexes had not answered. Although several examples of molybdenum terminal
References begin on page 65
----P
Chapter1
Table 1.5. Selected bond lengths and bond angles for 4.
Bond Lengths (A)
N(5A)-N(6A) 1.085(5)
N(5B)-N(6B) 1.083(6)
Mo(1)-N(5A) 1.990(4)
Mo(2)-N(5B) 1.995(4)
Mo(1)-N(4A) 2.197(3)
Mo(2)-N(4B) 2.183(4)
Mo(1)-N(1A) 1.989(4)
Mo(1)-N(2A) 1.995(4)
Mo(1)-N(3A) 1.994(4)
Mo(2)-N(1B) 2.000(4)
Mo(2)-N(2B) 1.994(4)
Mo(2)-N(3B) 1.986(4)
Bond Angles (deg)
Mo(1)-N(5A)-N(6A) 179.1(4)
Mo(2)-N(5B)-N(6B)
179.4(4)
N(4A)-Mo(1)-N(5A) 179.36(14)
N(4B)-Mo(2)-N(5B)
179.1(2)
Mo(1)-N(1A)-Si(1A) 127.3(2)
Mo(2)-N(1B)-Si(1B) 128.1(2)
N(1A)-Mo(1)-N(2A) 118.4(2)
N(1B)-Mo(2)-N(2B) 120.6(2)
Dihedral Angles (deg)a
-175.49
N(4A)-Mo(1)-N(2A)-Si(2A) 180.00
N(4A)-Mo(l)-N(3A)-Si(3A)
177.74
N(4B)-Mo(2)-N(1B)-Si(1B) 174.12
N(4B)-Mo(2)-N(2B)-Si(2B)
173.10
N(4B)-Mo(2)-N(3B)-Si(3B)
N(4A)-Mo(1)-N(1A)-Si(1A)
173.37
aObtained from a Chem-3D Drawing
dinitrogen complexes of the type Mo(N2)2(phosphine) 4 have been structurally characterized, 7 4 is
the first example of a terminal dinitrogen complex containing molybdenum in a relatively high
oxidation state. Recently, it has been shown that Mo[N(R)Ar] 3 (R = C(CD 3 )2 CH 3 , Ar = 3,5C 6 H3 Me2 ) can homolytically cleave dinitrogen to give the nitrido complex NMo[N(R)Ar]3.
13 , 14
Although the thermally-unstable, bridging dinitrogen complex (j-N2){Mo[N(R)Ar] 3 2 can be
observed spectroscopically, evidence for the formation of the terminal dinitrogen complex
(N2 )Mo[N(R)Ar] 3 is lacking. We speculate that the presence of the nitrogen donor in [N3 N]Mo
References begin on page 65
33
Chapter 1
destabilizes dz2 more than dxz or dyz resulting in a low spin configuration for [N3N]Mo and that
such a spin state would appear optimal to bind dinitrogen.
Synthesis of a Homobimetallic Bridging Dinitrogen Complex
When a toluene solution of 4 is heated to 84 'C under dinitrogen, a dramatic color change
from orange-red to royal purple is observed to occur over the course of 40 h as [N3 N]Mo-N=NMo[N 3 N] (5) is formed (equation 8). We propose that 5 forms by loss of dinitrogen from 4 to
give the unobserved trigonal monopyramidal species "[N3 N]Mo" which is trapped by a second
molecule of 4 to yield 5. 5 can be isolated as black microcrystals in 78% yield. The 1H NMR
spectrum of paramagnetic 5 in C6 D6 exhibits three broad, shifted resonances consistent with a
species in which the [N3 N]Mo portion is C3-symmetric. The UV-visible spectrum of 5 in pentane
has an intense absorption at 542 nm (E = 17,872 M- 1 cm-1).
[N 3N]Mo(N 2 )
4
toluene, 84 'C
-N 2
[N 3N]Mo-N=N-Mo[N 3N]
5
(8)
SQUID magnetic susceptibility data for solid 5 is plotted versus temperature in Figure 1.4
and can be fit to the Curie-Weiss law (x = J2/8(T-0)) over the temperature range 50-300 K to yield
g = 3.24(2) gB, 0 = -1.1(5) K. The value for g is close to the spin-only value for a system
containing two unpaired electrons (2.83 gB), as predicted on the basis of 5 being a Mo(IV)
diazenido complex analogous to {[(tBuMe 2 SiNCH 2 CH 2 )3 N]Mo }2 (g-N2 ).24 For complexes such
as 5, one can construct two sets of degenerate n orbitals from the dxz and dyz orbitals on
molybdenum and the Px and py orbitals on the nitrogen atoms, into which are placed 10 electrons
(3e from each Mo center and 4e from the 7t system of dinitrogen) resulting in two unpaired
spins. 22 Such a picture is consistent with the magnetic susceptibility data.
References begin on page 65
Chapter1
Figure 1.4. Plot of Xm (corrected for diamagnetism using Pascal's constants) versus T for
[N3 N]Mo-N=N-Mo[N 3 N] (5).
.. .. .. . ............................
0 .1
..................
........ ......
0.08
..
.r..
..
.
0
0O
00
0
X
m
Oi
0.04
0.02
000
I
I
0
0
50
100
0 oooo
0: 0 q0
ooo
150
200
250
300
350
T (K)
A toluene-d8 solution of 5 shows no signs of decomposition upon being heated to 105 'C
under dinitrogen for 72 h. In particular 5 is stable toward decomposition to [N3 N]MoN (see
below) via N-N bond homolysis. Unlike [N3NF]Mo-N=N-Mo[N 3 NF] 22 which can be reduced by
sodium amalgam to yield [N3NF]Mo(N2) {Na(ether)x}, 5 is not reduced by magnesium to give 1.
Functionalization of Dinitrogen
1 reacts cleanly with two equivalents of TMSC1 to give [N3N]MoN 2 TMS (6) as a yellow,
pentane-soluble solid in 88% yield (equation 9). However, we have found that 1 need not be
isolated and diamagnetic 6 can be obtained in high yield from the reduction of [N3 N]MoCl by
magnesium powder in the presence of TMSC1. The IR spectrum of 6 has a strong broad stretch at
1712 cm-1 that shifts to 1654 cm - 1 in 6- 15 N2 (Table 1.1). The
15 N
NMR spectrum of 6- 15 N2 in
C6 D6 consists of a pair of doublets at 356.9 and 238.1 ppm (JNN = 12 Hz). For comparison, in
References begin on page 65
35
Chapter1
[N3NF]Mo-N=N-Si(iPr3)
22
VNN is found at 1687 cm- 1 and the
15N
NMR spectrum exhibits
doublets at 366 and 228 ppm (JNN = 15 Hz).
{[N 3N]Mo-N=N} 2Mg(THF) 2 + 2 TMSC1
1
THF ,
2 [N3N]MoN2TMS + MgCl 2
(9)
(9)
6
1 also reacts cleanly with TMSOTf to give 6. However, when other electrophiles are used
the reactions are more complicated and product mixtures result. For example, reaction of MeOTf
with 1 yields two diamagnetic products along with 4. The IR spectrum of the product mixture has
a strong stretch at 1713 cm-1 and so one of the diamagnetic products is tentatively formulated as the
methyl analog of 6. However, the nature of the supporting ligand in such circumstances cannot be
stated with certainty in view of the tendency for a relatively small electrophile such as methyl to add
to amido nitrogens in the triamidoamine ligand (see 7 and 8 below). 1 reacts with 2 equivalents of
Mel to give a mixture of unidentified diamagnetic and paramagnetic species (according to 1H NMR
spectroscopy). If an excess of Mel is used the main product that can be identified by 1H NMR
spectroscopy is [N3 N]MoI.
In [N3NF]Mo complexes, efforts to reduce the dinitrogen ligand to the hydrazido level
were unsuccessful and it was found that [N3NF]Mo-N=N-Si('Pr 3 ) does not react with dihydrogen,
hydrazine or lithium aluminum hydride. 22 Drawing on these results, efforts to functionalize the
dinitrogen ligand in 6 have consisted of attempts to alkylate 6. Complex 6 does not react with
TMSOTf, TMSI or Mel. However, it does react with excess MeOTf (4 equivalents) in toluene
over 12 h to give a mixture of two diamagnetic products, 7 and 8, typically in a ratio of 1:3
according to 1 H NMR spectroscopy (equation 10). These two products can be separated by
fractional crystallization. Cooling THF/ether solutions of the product mixture yields the Mo(VI)
dimethylhydrazido complex, {[N(CH2CH2NSiMe3) 2 (CH 2 CH 2 NCH 3 )]MoN 2 (CH 3 )2 I +OTf- (7)
as an orange, crystalline solid in 20% yield. 7 is insoluble in pentane and slowly oils out of
benzene and toluene. The
References begin on page 65
19 F
NMR spectrum of 7 in C6 D6 reveals a singlet at -77.3 ppm for the
Chapter1
triflate ion. The 1H NMR spectrum of 7 in THF-d 8 has multiple resonances for the methylene
protons of the ligand backbone, consistent with the fact that it is not a C3-symmetric species. A
singlet at 3.73 ppm integrates for 3H and is assigned to the amido methyl group. A singlet at 3.72
ppm integrates for 6H and is assigned to the methyl groups of the hydrazido ligand. The presence
TMS
+
H3C\ NCH3
+
N
N
I
TMS
N
TMS
II
toluene
[N3 N]MoN 2TMS + 4 MeOTf-20 C - r.t.
/CH
TMSN.Mo -N
N
3
OTf -
+
TMS
11
N
.MoN
CH3
Tf
8
7
(10)
of a plane of symmetry in 7 is confirmed by the
13 C
NMR spectrum which exhibits four
resonances for the methylene backbone carbons of the ligand. The
15 N
2
15N
NMR spectrum of 7-
in THF-d8 consists of a pair of doublets at 374.8 and 157.2 ppm attributed to NX and Np of
the hydrazido ligand, respectively. 25 The upfield shift of the resonance attributed to Np compared
to the chemical shift of NP in 6 is consistent with the formulation of 7 as a hydrazido complex.
Efforts to obtain a solid state IR spectrum of 7 were thwarted by its apparent aversion to Nujol and
a vNN peak could not be assigned in an IR spectrum obtained in THF. 7 is thermally stable and a
THF-d8 solution of 7 shows no signs of decomposition on heating to 74 'C for 12 h.
Single crystals of 7 suitable for an X-ray study were grown from THF/pentane solutions at
-20 'C. Crystallographic data and collection and refinement parameters are given in Table 1.6.
The molecular structure of 7 (two views) along with the atom-labeling scheme is shown in Figure
1.5 while selected bond lengths and bond angles are listed in Table 1.7. Identification of 7 as a
cationic, dimethyl hydrazido complex is consistent with the observed structure.
References begin on page 65
1
--lslsl--- -~----"--~.;.
..~-~1
1
~ ~1111--.-------.~ ~....~1
-L-~~~ I--YI--3..~
I
.-.--PIII
r
CC-C-C~-P-C-PIII--~II^~ ~1LI11
ChapterI
Figure 1.5. Two views of the structure of {[Me-N 3 N]Mo=N-NMe 2 }OTf (7) with the triflate
ion omitted for clarity.
C(8)
C(7)
C (7)
C(7)
N(3)
References begin on page 65
Chapter1
Table 1.6. Crystallographic data, collection parameters and refinement parameters for 7 and 8.
Empirical Formula
C 16 H39 F3MoN 6 0 3SSi 2
C 18 H 4 5 F 3 MoN 6 0SSi 3
Formula Weight
604.71
662.87
Diffractometer
Siemens SMART/CCD
Siemens SMART/CCD
Crystal Dimensions (mm)
0.39 x 0.18 x 0.18
na
Crystal System
Orthorhombic
Orthorhombic
Space Group
Pbca
Pbca
14.723(3)
15.610(3)
b (A)
14.417(3)
12.478(3)
c (A)
26.243(4)
34.085(5)
c(0)
90
90
3 (0)
90
90
Y (0)
90
90
V (A3), Z
5571(2), 8
6639(2), 2
Dcalc (Mg/m3)
1.442
1.326
Absorption coefficient (mm-l)
0.679
0.611
Fooo
2512
2768
Temperature (K)
183(2)
183(2)
( range for data collection (0)
1.55 to 20.00
1.19 to 20.00
Reflections collected
15181
18299
Unique Reflections
2592
3088
R
0.0851
0.0452
Rw
0.1005
0.0531
GoF
1.293
0.855
References begin on page 65
Chapter1
Table 1.7. Selected bond lengths and bond angles for 7.
Bond Lengths (A)
Mo(l)-N(5)
1.747(10)
Mo(1)-N(1)
1.960(9)
Mo(1)-N(2)
1.950(9)
Mo(1)-N(3)
1.962(9)
Mo(1)-N(4)
2.235(9)
N(5)-N(6)
1.334(13)
Bond Angles (deg)
Mo(1)-N(5)-N(6)
173.6(8)
Mo(1)-N(1)-Si(1)
129.5(5)
Mo(1)-N(3)-C(31)
128.5(8)
Mo(1)-N(2)-Si(2)
125.1(5)
N(5)-N(6)-C(7)
119.1(10)
N(5)-N(6)-C(8)
115.9(10)
N(1)-Mo(1)-N(5)
103.5(4)
N(3)-Mo(1)-N(5)
94.5(4)
Dihedral Angles (deg)a
N(4)-Mo-N(1)-Si(1)
175.3
N(4)-Mo-N(2)-Si(2)
165.0
aObtained from a Chem-3D Drawing
An important feature of 7 is that one of the TMS groups of the ligand has been replaced by
a methyl group giving rise to a molecule with Cs symmetry, consistent with the NMR data. This
result also illustrates the susceptibility of Si-N bonds to undergo cleavage reactions leading to
ligand degradation. Such decomposition pathways are believed to contribute to the low yield of
[N3 N]MoC1 from the reaction of Li 3 N3 N with MoC14 (THF)2. 2 1 The Mo-N(5)-N(6) linkage in 7
is essentially linear (173.6(8)0) and the N-N bond length (1.334(13) A) is indicative of a bond
order of -1.5. Hence, the dinitrogen ligand in 7 is considerably reduced compared to structurallycharacterized diazenido complexes such as 1, [Mo(t-BuMe 2 SiNCH 2 CH 2 ) 3N]2(g-N 2 )24 (N-N =
1.20(2) A) and [N3 NF]MoN2SiiPr322 (N-N = 1.20(1) A). The N-N bond length in 7 falls within
References begin on page 65
Chapter1
32 3 6
the range reported for other molybdenum and tungsten hydrazido complexes (1.28 - 1.39 A). -
Also, the short Mo-N(5) bond length (1.747(10)
A) is
consistent with an increase in the multiple
bonding between the metal center and Na. N(6) is a planar nitrogen, the sum of the angles being
3530 . This planarity allows for delocalization of the nitrogen lone pair throughout the Mo-N-N nt
system. The molybdenum atom is displaced from the plane defined by the amide nitrogens by
0.369
A in
the direction of Na. The methyl groups of the hydrazido ligand point between the
methylated ligand arm and a TMS-substituted ligand arm as opposed to pointing between two
TMS-substituted ligand arms presumably for steric reasons. The NNMe 2 moiety is tilted -10'
toward the methylated ligand arm (N(5)-Mo-N(3) = 94.5(4)0, N(5)-Mo-N(1) = 103.5(4) ° , N(5)Mo-N(2) = 104.6(4)0). Both features can be attributed to greater steric demands of Si(1) and Si(2)
compared to a methyl group. We can be certain that there is no proton present on N(3) in view of
the virtual identity of the Mo-N(1), Mo-N(2), and Mo-N(3) bond lengths and the fact that they are
all similar to Mo-Namido bond lengths in many other triamidoamine complexes.17
The
second
product
of
the
reaction
between
6
and
MeOTf,
{[(Me 3 SiNCH 2 CH 2 )2 NCH 2 CH 2 NMe 2 ]Mo-N=NSiMe 3 }OTf (8), can be isolated in 35% yield as
red crystals from THF/pentane solutions at -20 OC. The solubility properties of 8 are consistent
with it being a cationic species. A singlet at 0.34 ppm in the 1 H NMR spectrum of 8 in THF-d8
integrates as 27 protons and is assigned to the three TMS groups, the resonances for which are
apparently accidentally equivalent. (In C6 D6 two resonances are observed at 0.29 ppm (18H) and
0.16 ppm (9H)). A second singlet at 2.76 ppm integrates as 6 protons but is 1 ppm upfield of the
methyl amido protons of 7. There are six sets of multiplets for the twelve methylene protons of the
ligand backbone and so 8 is not a C3-symmetric complex. The
13 C
NMR spectrum of 8 reveals
TMS groups in two different environments and four methylene carbon resonances suggesting the
presence of a plane of symmetry in 8. The IR spectrum of 8 has a strong broad stretch at 1724
cm - 1 that shifts to 1668 cm - 1 in 8- 15 N2 and the
15 N
NMR spectrum reveals resonances at 361.5
and 244.3 ppm (JNN = 13 Hz) (Table 1.1). The position of vNN and the downfield shift of Np in
the
15 N
NMR spectrum are suggestive of a diazenido complex (compare with 1 and 6 above) but
References begin on page 65
I
I
I
~l~r~
L
Chapter1
the complexity of the 1 H and
13 C
NMR spectra precluded a determination of the molecular
structure of 8 so an X-ray study was carried out to resolve the issue.
X-ray quality crystals of 8 were obtained by crystallization from THF/pentane at -20 'C.
Crystallographic data and collection and refinement parameters are given in Table 1.6. The
molecular structure of 8 along with the atom-labeling scheme is shown in Figure 1.6 while selected
bond lengths and bond angles are listed in Table 1.8.
Figure 1.6. View of the structure of {[(Me3SiNCH2CH2)2NCH2CH 2 NMe 2 ]MoN 2 TMS }OTf
(8) with the triflate ion omitted for clarity.
Si(3)
N(6)
N(5)
Two features of 8 are immediately apparent from Figure 1.6 and both serve to illustrate the lack of
selectivity of the methylation reaction. Firstly, the diazenido ligand has emerged unscathed from
the reaction and so further functionalization of dinitrogen has not been achieved. Secondly, one of
the ligand arms has been doubly methylated thereby converting the triamidoamine ligand into a
diamidodiamine ligand. This conversion explains the upfield shift of the methyl protons of 8
References begin on page 65
ChapterI
relative to those of 7. The Mo-N(3) bond length at 2.181(5) A is typical of a dative amine bond
and should be compared with the Mo-N(4) bond length of 2.229(6) A. The dinitrogen bond length
of 1.206(9) A is within the range of N-N bond lengths of other crystallographically characterized
diazenido complexes of the TMS-TREN system. 22 ,24 The Mo-N(5)-N(6) and N(5)-N(6)-Si(3)
linkages are essentially linear with angles of 172.80 and 170.50 respectively.
Table 1.8. Selected bond lengths and bond angles for 8.
Bond Lengths (A)
Mo(1)-N(5)
1.803(7)
Mo(1)-N(1)
1.958(5)
Mo(1)-N(2)
1.950(5)
Mo(1)-N(3)
2.181(5)
Mo(l)-N(4)
2.229(6)
N(5)-N(6)
1.206(9)
N(1)-Si(1)
1.744(6)
N(2)-Si(2)
1.745(6)
N(6)-Si(3)
1.670(9)
Bond Angles (deg)
N(5)-Mo(1)-N(3)
95.2(2)
N(5)-Mo(1)-N(1)
100.3(3)
N(5)-Mo(1)-N(2)
100.8(3)
N(2)-Mo(1)-N(3)
116.5(2)
N(3)-Mo(1)-N(4)
80.2(2)
N(6)-N(5)-Mo(1)
172.8(7)
N(5)-N(6)-Si(3)
170.5(8)
N(4)-Mo(1)-N(5)
175.4(2)
Mo(1)-N(1)-Si(1)
126.2(2)
Mo(1)-N(2)-Si(2)
125.8(3)
Dihedral Angles (deg)a
N(4)-Mo(1)-N(1)-Si(1)
179.4
aObtained from a Chem-3D Drawing
References begin on page 65
N(4)-Mo(1)-N(2)-Si(2)
173.0
Chapter1
Attempts have been made to improve the selectivity of the reaction shown in equation 10.
If the reaction mixture is maintained at -20 'C, no reaction is observed over the course of 18 h and
6 is recovered. If less than 4 equivalents of MeOTf are used, the reaction does not go to
completion (according to 1H NMR spectroscopy). The reactivity of 1 with electrophiles such as
Mel and MeOTf has been explored briefly in hopes of synthesizing hydrazido complexes in one
step without isolation of the intermediate diazenido complexes. However, such reactions give rise
to complex mixtures of diamagnetic and paramagnetic products and so this approach was
abandoned.
The proposed mechanism for the formation of 7 and 8 is shown in Scheme 1.2. In the
first step a methyl electrophile reacts with 6 by attacking an amido group of the ligand to give A.
Loss of TMSOTf from A yields B. Sterically, the methylated amido nitrogen of B is probably
more accessible than Np of the diazenido ligand and further alkylation at the equatorial nitrogen
would then produce 8. Reaction of 8 with methyl triflate is unlikely due to its cationic nature
which perhaps accounts for its isolation. In a competing reaction, alkylation at Np of B yields C
which loses TMSOTf to give D. Further alkylation of D could occur at the methylated amido
nitrogen to produce E or at Np, to yield 7. In triamidoamine complexes one of the three linear
combinations of p orbitals on the equatorial amido nitrogens is of A2 symmetry and in the C3 v
point group there is no metal-based orbital of matching symmetry.
Presumably, it is the
availability of this ligand-centered, nonbonding orbital that leads to alkylation of the equatorial
amido nitrogen in 6, B and D. The low yields of 7 and 8 suggest that competing pathways such
as those that lead to E are accessible although products arising from such reactions have not been
isolated. It should be noted that diazenido complexes of the type M-N=N-Me have not been
isolated in any TREN-based systems. The reasons for this are not clear although the smaller size
of a methyl group compared to a trimethylsilyl group may result in such species being prone to
intermolecular decomposition. Whether the proposal outlined above is correct or not, it is clear that
alkylation at the equatorial nitrogen competes with alkylation of Np of the diazenido ligand at least
when strong electrophiles such as methyl triflate are employed.
References begin on page 65
Chapter1
Scheme 1.2. Proposed mechanism for the formation of 7 and 8.
TMS
TMS
N
N
II
N
II
MeOTf
TMS
Mo -N
TMS
N
S MMN
~~Mo -* N
OTf-
6
- TMSOTf
TMS
TMS
N
N
N
N
N
N
TMS
Me
II
MeOTf
Mo-N
II
I
Me
Mo -N
OTf
MeOTf
ON
Mo-a-
B
Me
N
IOTf
8
TMSOTf
-Me
--
Me
N
N
II
N
Mo -N
MeOTf
Me
N
II
N
Me
[ Me
Mo---N
E
References begin on page 65
+
] OTf
Me
Me
I
MeOTf
N
II
Mo -N
/
I
Me
OTf-
Chapter 1
Having isolated and characterized 7 and 8 we began to explore their reactivity with a view
to further functionalizing the dinitrogen ligand. In particular we examined the reactivity of 7 and 8
with MeMgCl in order to determine whether an alkyl nucleophile would add to No, to NP, or to the
metal center.
8 reacts instantaneously with MeMgC1 in THF to give diamagnetic
[N(CH 2 CH 2NSiMe 3 )2 (CH 2 CH 2NMe 2 )]Mo(N 2 TMS)(Me) (9) (equation 11). Resonances at
TMS
TMS
TMS
N
II
N
TMS
THF
CH3
, Mo*NCH3
oTMS
TMS
OTf
+ MeMgC1
TMS lr.M..
N
II
N
o
- Me
Me
N(CH 3 )2
8
9
(11)
0.47 ppm in the 1H NMR spectrum and at 23.9 ppm in the
13 C
NMR spectrum of 9, taken in
C 6 D 6 , suggest that alkylation has occurred at molybdenum. In the 1H NMR spectrum the
resonance due to the methyl groups of the amine ligand arm appears at 1.90 ppm which is
somewhat upfield of the corresponding resonance in 8 (2.76 ppm). The IR spectrum of 9 in
Nujol shows a strong, broad absorption at 1640 cm- 1 (1577 cm- 1 in 9-15N2) while the
15N
NMR
spectrum taken in C6 D6 consists of two doublets at 374.6 and 239.5 ppm (JNN = 15 Hz) (Table
1.1). 9 decomposes to a black, oily solid upon prolonged exposure to vacuum (1 h). Satisfactory
elemental analysis was obtained for a sample of 9 that was subjected to vacuum drying for - 10
min. Although these data do not reveal whether the amine donor is still bound to the metal or not,
an X-ray structure revealed that it is not.
Single crystals of 9 were grown from saturated hexamethyldisiloxane solutions at -20 'C.
The molecular structure of 9 along with the atom-labeling scheme is shown in Figure 1.7 while
References begin on page 65
ChapterI
selected bond lengths and bond angles are listed in Table 1.9. Crystallographic data and collection
and refinement parameters are given in Table 1.10. The structure of 9 confirms that alkylation has
occurred at molybdenum and that the amine ligand arm has dissociated to yield a trigonal
bipyramidal environment around the metal center. The Mo-N(5)-N(6) linkage is linear and the
N(5)-N(6) bond length at 1.229(3) A is consistent with 9 being described as a diazenido complex.
The diazenido ligand in 9 in quite bent at the
f3
nitrogen, with N(5)-N(6)-Si(2) = 137.0(2)0
(compare with the corresponding angle in 8 which is essentially linear at 170.5(8)0). We attribute
the bending of the diazenido ligand to an absence of three approximately equal steric interactions
that would oppose bending, rather than to electronic effects. The environment above C(7) is
relatively open, so the trimethylsilyl group of the diazenido ligand bends away from the bulky
TMS groups on the equatorial nitrogens toward C(7). A comparison of the
15 N
NMR chemical
shifts and the IR stretches for the diazenido ligands in 1, 6, 8 and 9 suggests that in triamidoamine
complexes of the type being investigated here,
15 N
NMR and IR data are not reliable parameters on
which to base any conclusion as to whether the diazenido ligand is significantly bent at Np or not in
the solid state (Table 1.1).37
Table 1.9. Selected bond lengths and bond angles for 9.
Bond Lengths (A)
Mo(1)-N(5)
1.789(2)
Mo(1)-N(1)
1.983(2)
Mo(1)-C(7)
2.153(3)
Mo(l)-N(3)
1.981(2)
Mo(1)-N(4)
2.312(2)
N(5)-N(6)
1.229(3)
N(6)-Si(2)
1.726(2)
Bond Angles (deg)
N(5)-Mo(1)-N(1)
98.65(8)
N(3)-Mo-(1)-N(1)
123.99(9)
N(5)-Mo(1)-C(7)
91.69(10)
N(3)-Mo(1)-C(7)
114.47(11)
C(4)-N(4)-Mo(1)
117.34(14)
N(6)-N(5)-Mo(1)
179.5(2)
N(5)-N(6)-Si(2)
137.0(2)
References begin on page 65
Chapter 1
Table 1.10. Crystallographic data, collection parameters and refinement parameters for 9.
Empirical Formula
C 18H4 9 MoN 6 Si 3
Formula Weight
529.84
Diffractometer
Siemens SMART/CCD
Crystal Dimensions (mm)
0.50 x 0.40 x 0.40
Crystal System
Monoclinic
Space Group
P21/c
10.235(3)
b (A)
14.315(7)
c (A)
20.277(7)
a (0)
90
103.80
7(0)
90
V (A3), Z
2885(2), 4
Dcale (Mg/m3 )
1.220
Absorption coefficient (mm- 1)
0.594
1132
Temperature (K)
183(2)
E range for data collection (0)
1.76 to 23.28
Reflections collected
11505
Unique Reflections
4130
R
0.0251
Rw
0.0272
GoF
1.026
References begin on page 65
i --1
'~ - I
1~ "--~---9--
-- ---I
II-
--
-- ----
-
-I'-1
----
------
Chapter1
Figure 1.7. A view of the structure of 9 with the triflate ion omitted for clarity.
Si(1)
Addition of MeMgCl to a THF solution of 7 at -20 OC results in an immediate color change
to blood red. The 1H NMR spectrum of the product (10) in C6 D6 is shown on the lower half of
Figure 1.8. This spectrum is consistent with 10 being a complex of low symmetry. There are 10
sets of resonances for the 12 methylene protons of the ligand backbone and the singlet at 0.18 ppm
indicates that methylation has occurred at molybdenum (cf. 0.74 ppm in 9). Both the 1H and
13 C
NMR spectra of 10 have two resonances for the two TMS groups on the ligand which is also
consistent with a complex of low symmetry. Two sets of doublets comprise the
15 N
NMR
spectrum of 10- 15 N2 with the resonance attributed to Np appearing at 142.0 ppm. With these
data in hand we formulate 10 as a pseudo-octahedral methylhydrazido complex shown in equation
References begin on page 65
-
-----.
Chapter 1
12, alkylation between TMS- and Me-substituted amido nitrogens being the sterically more
accessible position.
H3 C\ /CH
+
CH 3
H 3C
3
N
TMSN
TMS M
jjII
**I
Mo-N
/H
3
TMS
T
N
. ....
Me
M6
CH
N
N
sNN
7
10
(12)
Efforts to crystallize 10 have been hampered by its high solubility in pentane and its
thermal instability even at room temperature. Over 24 h, a C6 D6 solution of 10 changes color
from blood red to orange-brown. This transformation is accelerated by heating a sample to 65 °C.
If the solvent is removed and the residue is extracted with pentane, a pale yellow, crystalline solid
(11) can be isolated. The 1H NMR spectrum of 11 is shown on the upper half of Figure 1.8. It is
apparent from this spectrum that 11 is a complex of higher symmetry than 10. There is a single
resonance for the TMS groups on the ligand at 0.59 ppm and three sets of multiplets for the
methylene protons (3.25, 2.78 and 2.21 ppm), consistent with a complex possessing mirror
symmetry. The singlet at 4.08 ppm is assigned to the methyl group on the triamidoamine ligand
arm. The
13 C
NMR spectrum of 11 exhibits four resonances for the methylene carbons of the
ligand backbone and a single resonance for the TMS groups of the ligand, suggesting that 11
contains a plane of symmetry. Spectroscopic and elemental analysis data support the formulation
of 11 as the nitride complex [Me-N 3 N]Mo-N (equation 13). The 1H NMR spectrum of 11 is a
hybrid of that of [N3 N]Mo=N (see below) and that of [(MeNCH 2 CH 2 )3 N]Mo-N. 38 The IR
spectra of 11 and 11- 15 N2 are superimposable except for a single absorption that occurs at 1002
cm - 1 in the spectrum of 11 and shifts to 977 cm - 1 in the spectrum of 11- 15 N2 , characteristic of a
References begin on page 65
I
I
I
4.0
I
I
I
I
I
3.5
I
I
I
I
I
3.0
I
i
2.5
I I I
I
I
2.0
1.5
Figure 1.8. 1H NMR spectra of 10 (lower spectrum) and 11 (upper spectrum) in C6 D6 -
I I
1.0
I
I
I
0.5
I
I
I
ppm
I
Chapter1
M-N triple bond stretch. 39 Furthermore, the
15N
NMR spectrum of 11- 15 N2 consists of a singlet
at 866.1 ppm which is also indicative of a metal nitride complex. 39
H3c
.CH
N
TMS
3
N
IIC
N,
TMS
Mo"
NC
NM
N
65 C
MCH ]
TM
NI-.. MO-
6D6
I
3
/CH3
N
f\
+ CH4 + (CH 3)2NH
10
11
(13)
The thermolysis of 10 has also been carried out in THF-d8 with essentially the same result.
The yield of 11 (versus an internal standard) is 67% although the isolated yield (30%) is lowered
due to its solubility in pentane. Among the organic products are methane (16% in solution) and
dimethylamine (38% in solution), identified by comparison with 1H and
13 C
NMR spectra of
authentic samples and measured via an internal standard. Resonances at 0.19 ppm in the 1 H NMR
spectrum and at -1.22 ppm in the
13 C
NMR spectrum are assigned to CH 4 . The 1H NMR
spectrum also exhibits a doublet at 2.32 ppm (3 JHH = 6 Hz) attributed to (CH 3 ) 2 NH. If the
thermolysis is carried out employing 10- 15 N 2 in THF-ds, a doublet is observed at 10.74 ppm
( 1JNH = 69 Hz) in the
15 N
NMR spectrum consistent with the formation of (CH 3 )2 15 NH.
However, it is clear from the complexity of the NMR spectra of decomposed 10 that a fraction of
the triamidoamine ligand has been attacked in some significant manner as evidenced in the 1H
NMR spectrum by numerous resonances between 0.4 and 0.0 ppm. We speculate that the
decomposition of 10 to 11 proceeds initially via Mo-C bond homolysis to produce a methyl radical
and the unstable Mo(V) species [Me-N 3 N]Mo=N-N(CH 3 )2 which decomposes via homolytic N-N
bond cleavage to give 11 and a dimethylamine radical. However, it is equally likely that the initial
step may involve N-N bond homolysis followed by scission of the Mo-C bond. There is no
References begin on page 65
ChapterI
evidence for the incorporation of deuterium in the organic products that have been identified. This
suggests that hydrogen atom abstraction from the [N3 N] 3 - ligand may be occurring and could
explain the observed ligand degradation. Unfortunately, the long and low yield route to 10 has
rendered a detailed study of its decomposition impractical. However, the progress we have made
with regard to the functionalization of dinitrogen in [N3 N]Mo complexes and the difficulties that
we have encountered with the lability of TMS groups in the [N3N] 3 - ligand have prompted us to
revisit the [N3NF] 3- system and preliminary results suggest that this approach will be fruitful. For
example, in a reaction analogous to that which yields 7 (see above), [N3NF]Mo-N=N-SiMe 3 has
been found to react cleanly with MeOTf to give {[N3NF]Mo=NNMe
2
}OTf. 4 0
{ [N3NF]Mo=NNMe2}OTf can be alkylated by MeMgC1 and the resulting complex does
decompose upon heating to give [N3NF]MoN as one of the products. 40 These results suggest that
we will be able to establish the mechanism or mechanisms by which species analogous to 10
decompose by N-N bond cleavage.
Having functionalized dinitrogen to the nitrido stage we were compelled to explore the
chemistry of such nitride complexes since in a catalytic cycle the second nitrogen must be removed
from the metal center in order to regenerate a [N3N]MoX complex which could then be reduced
under dinitrogen to begin the cycle again. Since 11 is only available to us in low yield, we set out
to synthesis [N3 N]MoN (12) by more direct routes. Although [N3 N]WN 19 can be synthesized
from [N3 N]WCl and NaN 3 , the analogous reaction with [N3 N]MoCl does not yield [N3N]MoN in
any appreciable amount. [N3 N]MoCl does react with TMSN 3 at elevated temperatures and 12 can
be isolated from the reaction as a yellow, crystalline solid in 88% yield (equation 14). Presumably
an azide complex is formed as an intermediate in this reaction and although decomposition of azide
[N3N]MoC1 + 4 TMSN 3
References begin on page 65
toluene
90C, 24
90 C, 24 h11
[N3N]MoN
11
(14)
Chapter1
complexes is a common method for preparing nitride complexes, isolation of intermediate
organoazide complexes has been documented in only a few cases. 4 1,42 The 1H NMR spectrum of
12 taken in C6 D6 is typical of C3-symmetric complexes of this type and consists of a singlet for
the TMS groups of the ligand at 0.56 ppm and a pair of triplets at 3.23 and 2.14 ppm assigned to
the methylene protons of the ligand backbone. The IR spectrum of 12 has a strong absorption at
1001 cm- 1 that is assigned to a M-N triple bond stretch and should be compared with that of 11.
12 reacts readily with MeOTf or TMSOTf in toluene to give the corresponding cationic imido
complexes, { [N3N]MoNMe) OTf (13) and {[N3 N]MoNTMS }OTf (14) respectively (equation
15). Both 13 and 14 are isolated in high yield and have solubility properties that are consistent
with their cationic nature e.g. both slowly oil out of toluene or benzene but are highly soluble in
THF.
[N3N]MoN + ROTf
12
toluene
{[N 3N]MoNR)OTf
(15)
13 (R = Me)
14 (R = TMS)
The chemistry of 13 and 14 has been explored by Dr. Klaus Wanninger. Both react
readily with MeMgCl yielding complexes arising from alkylation at molybdenum. Complex 14
can be reduced by Li2C 8 H8 or sodium naphthalenide to give the structurally-characterized Mo(V)
imido complex [N3 N]MoNTMS. Further details of this chemistry can be found in the literature.19
However, it is noted that efforts to remove the imido group from the metal center to generate a
[N 3N]MoX complex have been unsuccessful to date. An alternative approach to this problem may
involve reduction of 12 as the initial step although the cyclic voltammogram of 12 (obtained by
Dr. Luis Baraldo) exhibits a reduction wave at -2.9 eV (versus ferrocene/ferrocenium) indicating
that 12 is exceedingly difficult to reduce.
References begin on page 65
Chapter 1
DISCUSSION
Our exploration of the dinitrogen chemistry of [N3N]Mo complexes has proved to be a
fruitful one as evidenced by the plethora of complexes described above. The stepwise reduction
and functionalization of dinitrogen has been achieved and examples of terminal dinitrogen,
diazenido, hydrazido and nitrido complexes have been isolated. This work complements the
extensive work carried out on the functionalization of dinitrogen in [M(N 2 )2 (P) 4 ] (P = phosphine;
M = Mo, W) and [M(N 2 )2 (P-P)] (P-P = chelating diphosphine; M= Mo, W) complexes. 6- 9 A
facile entry to the chemistry is achieved by the reduction of [N3N]MoCl with magnesium powder
to give 1 in high yield. 1 is noteworthy for a number of reasons. Firstly, as noted above, it
provides us with an entry into dinitrogen chemistry in the TMS-TREN system. Secondly, in
previous work in our group, efforts to crystallize [N3NF]Mo(N2)[Na(ether)x] 22 were unsuccessful
and so 1 represents the first example of a structurally-characterized salt of a diazenido complex in
the TREN-based systems. Thirdly, oxidation of 1 affords the neutral terminal dinitrogen complex
4 in high yield. Finally, heterometallic dinitrogen complexes containing Zr, V and Fe can be
prepared by employing {[N3 N]Mo(N 2 )}- as a nucleophile in the form of 1 and the synthesis of
such complexes is the subject of Chapter 2.
Although the dinitrogen ligand in 4 is labile as suggested by exchange of 14 N2 with
15 N
2
and by the decomposition of 4 to yield 5, the "[N3 N]Mo" species formed by dissociation of
dinitrogen from 4 has not be observed. Such a low spin species is expected to be of high energy
and reactivity, the energy of the dz2 orbital being much higher than that of the dxz or dyz orbital as a
consequence of the presence of the apical amine donor. The high reactivity of such a species is
demonstrated by the isolation of [bitN 3 N]Mo, the product of C-H activation of one of the
trimethylsilyl groups of the ligand, isolated from the reduction of [N3N]MoCl by magnesium in the
absence of a donor ligand (see Chapter 3). In contrast to "[N3 N]Mo", Mo[N(R)Ar] 3 (R =
C(CD 3 )2 CH 3 , Ar = 3,5-C 6 H 3 Me 2 ) is isolable and has a high spin configuration. 14 We suspect
that this difference between the triamidoamine and trisanilide complexes is the crucial one that
differentiates their chemistry with dinitrogen.
References begin on page 65
Chapter1
Although 7 is only available in low yield, it does represent a significant breakthrough in the
context of the dinitrogen chemistry of TREN complexes being the first hydrazido complex to be
isolated in such systems. Aside from the low yield, a further drawback of the chemistry is that the
methylation reaction is non-selective and the TMS-TREN ligand has become involved in the
chemistry. It appears that the nucleophilicity of the 3 nitrogen in 6 is not sufficiently different
from that of the amido nitrogens to allow MeOTf to react preferentially with the diazenido ligand as
further demonstrated by the isolation of 8. However, 8 is an example of a new type of
diamido/bisdonor complex that may be of use in future research provided more direct ways can be
found to such ligands and complexes. The ability of the equatorial donor to associate and
dissociate from the metal center as required may be a key feature of such chemistry.
Reaction of 7 with MeMgCl results in nucleophilic attack at the metal center to generate a
methyl hydrazido complex, 10. This result is perhaps not surprising in light of the crystal
structure of 7.
The long N-N bond and shortened Mo-N bond are consistent with {[Me-
N3 N]Mo+=N-N(CH 3)2 }OTf as the major resonance structure for 7. The decomposition of 10 to
11 is also not surprising in view of the documented propensity for triamidoamine complexes to
form strong M-E bonds (E = CR, N, P, As).19-21,43 The intimate details of this decomposition are
not known at this stage and a complete investigation is hampered by the low yield of 7 in the
preceding step.
Despite the limitations of TMS-TREN as a robust ligand, the present study demonstrates
the feasibility of the stepwise reduction and functionalization of dinitrogen in molybdenum
triamidoamine complexes. We have been able to isolate and crystallographically characterize
several intermediates and the reduction of dinitrogen can be correlated with the N-N bond lengths
in these complexes. These results have prompted us to revisit the C6 F5 -TREN 22 system and it
appears likely that the salient features of the chemistry described herein will be transferable to such
a system without the possibility of the side-reactions that plague the TMS-TREN system.
References begin on page 65
Chapter1
EXPERIMENTAL PROCEDURES
General Details. All experiments were performed under a nitrogen atmosphere in a
Vacuum Atmospheres drybox or by standard Schlenk techniques unless otherwise specified.
Pentane was washed with sulfuric acid/nitric acid (95/5 v/v), sodium bicarbonate, and water,
stored over calcium chloride, and distilled from sodium benzophenone ketyl under nitrogen.
Toluene was distilled from sodium, and CH 2 CI2 was distilled from CaH 2 . Anhydrous diethyl
ether and THF were sparged with nitrogen and passed through alumina columns. 4 4
Hexamethyldisiloxane was purchased from Aldrich, dried over sodium and then vacuum
transferred into a small storage flask. All solvents were stored in the dry box over activated 4 A
molecular sieves.
NMR data were obtained at 300 or 500 MHz (1H), 75.4 MHz ( 13 C) and 50.7 MHz ( 15 N).
Chemical shifts are listed in parts per million downfield from tetramethylsilane for proton and
carbon.
15 N
chemical shifts are referenced to external CH 3 NO 2 whose shift is +380.2 ppm with
respect to liquid ammonia (taken as 0 ppm). Coupling constants are listed in Hertz. Spectra were
obtained at 25 *C unless otherwise noted. Benzene-d6 and toluene-d8 were pre-dried on CaH 2 ,
vacuum transferred onto sodium and benzophenone, stirred under vacuum for two days and then
vacuum transferred into small storage flasks and stored over molecular sieves. [N3 N]MoCl was
prepared as described in the literature. 2 1 TMSOTf, MeOTf, Mel, PdC12 (PPh3 )2 , NiC12 (PPh 3 )2 ,
magnesium powder and MeMgCI were purchased from commercial vendors and used as received.
ZnC12 was dried by heating to 80 'C for 24 h under active vacuum.
UV/visible spectra were recorded on a HP 8452 Diode Array spectrophotometer using a
Hellma 221-QS quartz cell (path length = 10 mm) sealed to a gas adapter fitted with a Teflon
stopcock. IR spectra were recorded on a Perkin-Elmer 1600 FT-IR spectrometer. Elemental
analyses (C, H, N) were performed in our laboratory using a Perkin-Elmer 2400 CHN analyzer or
by Microlytics Analytical Laboratories of Deerfield MA. X-ray data were collected on Siemens
SMART/CCD diffractometer and general experimental details are described in the literature. 45
References begin on page 65
Chapter1
SQUID Magnetic Susceptibility Measurements. Measurements were carried out
on a Quantum Design SQUID magnetometer. Data were obtained at a field strength of 5000
Gauss. Straws and gel caps (Gelatin Capsule No. 4 Clear) were purchased from Quantum Design.
The sample was prepared in the drybox by the following method. A gel cap and a square of
parafilm were weighed. The sample was placed in the gel cap and the parafilm inserted above it.
The gel cap was closed and the mass of the sample was ascertained by weighing the loaded gel
cap. The gel cap was placed in a straw which was then mounted on the sample rod and placed in
the magnetometer. Two runs were performed on the sample - one from 5 to 300 K and a second
from 300 to 5 K. Measurements were made at the following increments: 5-10 K (every 1 K), 1020 K (every 2 K), 20-50 K (every 3 K), 50-100 K (every 5 K), 100-200 K (every 10 K), 200-300
K (every 20 K).
{[(Me 3 SiNCH 2 CH 2 )3 N]MoN2} 2 Mg(THF)2 (1). [(Me 3 SiNCH 2 CH 2 ) 3 N]MoCl
(785 mg, 1.60 mmol) was dissolved in 30 mL of THF. Magnesium powder (100 mg, 4.16 mmol)
was added to the solution which was then stirred for 17 h. THF was removed in vacuo and the
residue was extracted with 30 mL toluene. 1,4-dioxane (560 mg, 6.37 mmol) was added and the
solution stirred for 30 min. MgC12 .(dioxane) was removed by filtration through a pad of Celite.
Toluene was removed in vacuo and the orange solid was crystallized from diethyl ether; yield 820
mg (90%).
1H
NMR(C 6 D6) 8 3.98 (m, 8H, THF), 3.60 (t, 12H, NCH 2 CH 2 N), 2.21 (t, 12H,
NCH 2 CH2N), 1.46 (m, 8H, THF), 0.61 (s, 54H, NSiMe 3 ).
13 C{ 1H)
NMR(C 6 D 6 ) 5 71.0
(THF), 54.7 (NCH 2 CH 2 N), 52.0 (NCH2 CH 2 N), 25.6 (THF), 4.8 (NSiMe 3 ). IR(THF, cm - 1)
1719 (N=N). Anal. Calcd. for C 38H9 4 N12 Si 6 02Mo 2 Mg: C, 40.18; H, 8.34; N, 14.80. Found:
C, 40.35; H, 8.13; N, 14.76.
1- 15 N2 . [(Me 3 SiNCH 2 CH 2 )3 N]MoCl (520 mg, 1.06 mmol) was dissolved in 10 mL of
THF and placed in a glass bomb with a stirring bar and an excess of magnesium powder. The
vessel was subjected to three freeze-pump-thaw cycles to remove any
14 N
2
present.
15 N
2
(1 atm)
was introduced and the solution stirred for 20 h. The product was isolated in a manner analogous
References begin on page 65
Chapter1
to that for {[(Me 3SiNCH 2 CH 2 )3N]MoN 2 12Mg(THF)2.
15N
NMR(C 6D6) 8 377.0 (JNN = 12),
304.4 (JNN = 12). IR(THF, cm -1) 1662 (N=N).
{[(Me 3 SiNCH 2 CH2) 3 N]MoN2}[Na(15-crown-5)]
(2). [N3N]MoCl (204 mg,
0.42 mmol) was dissolved in 5 mL of THF and cooled to -20 'C. Sodium naphthalenide (1654
ItL, 0.85 mmol) was added dropwise to the stirred solution. After 40 min the solvent was
removed under reduced pressure and the residue extracted with 7 mL diethyl ether. Removal of
NaC1 was effected by filtration through a pad of Celite. 15-crown-5 (83 jgL, 0.42 mmol) was
added to the ether solution which was chilled at -20 'C to afford the product as orange plates; yield
195 mg (64%).
1H
NMR(tol-d 8) 5 3.76 (t, 6H, NCH 2CH 2N), 3.10 (s, br, 15-crown-5), 2.27 (t,
6H, NCH2 CH 2 N), 0.70 (s, 27H, NSiMe 3).
13C{ 1H}
NMR(tol-ds) 8 69.0 (15-crown-5), 55.0
(NCH 2CH 2N), 52.2 (NCH 2CH 2N), 4.6 (NSiMe3). IR(Nujol, cm- 1) 1791(N=N).
[(Me 3 SiNCH2CH2) 3 N]Mo(N2) (4). Method 1. { [N3N]MoN 2 12Mg(THF)2 (302
mg, 0.27 mmol) was dissolved in 10 mL THF and the solution was cooled to -20 'C.
Pd(PPh3)2C12 (187 mg, 0.27 mmol) was added all at once as a solid to the stirred solution of
{ [N3N]MoN 2 12Mg(THF)2. Within one minute the solution had become deep green in color.
After 45 min the solvent was removed and the residue extracted with 40 mL of pentane. The
mixture was filtered through Celite and part of the pentane was removed in vacuo to yield red
crystals; yield 205 mg (80%).
Method 2. { [N3N]MoN212Mg(THF)2 (302 mg, 0.27 mmol) was dissolved in 10 mL THF and
ZnC12 (36 mg, 0.27 mmol) was dissolved in 2 ml THF. Both solutions were cooled to -20 °C and
the ZnC12 solution was added to the stirred solution of {[N3N]MoN 2 12Mg(THF)2. After 4 h the
solution was filtered through Celite and the solvent removed in vacuo. The residue was extracted
with 20 mL of pentane, filtered through Celite and dried in vacuo. Recrystallization from diethyl
ether afforded the product as red crystals; yield 181 mg (70%, 2 crops). 1H NMR (C6 D6) 8 14.02
(CH2), -4.53 (TMS), -40.57 (CH2). IR(Pentane) cm -1 1934 (N=N); IR(Nujol) cm - 1 1910, 1900
(N=N).
References begin on page 65
Chapter1
4- 15 N2. This compound was prepared in an analogous manner to 4 using 1- 15 N2: IR
(Pentane)
cm- 1 1870 (N=N); IR(Nujol) cm- 1 1846, 1839 (N=N).
Anal. Calcd. for
C 15 H3 9 N4 15 N2 Si3Mo: C, 37.09; H, 8.09; N, 17.71. Found: C, 37.08; H, 8.49; N, 17.50.
[N3 N]Mo-N=N-Mo[N3N] (5). [N3 N]Mo(N2) (200 mg, 0.41 mmol) was dissolved
in 5 mL of toluene and placed in a glass bomb along with a stirring bar. The bomb was sealed and
heated to 84 'C for 40 h. During this time the reaction mixture turned deep purple in color. The
toluene was removed in vacuo and the residue extracted with ether. Following filtration and
reduction the ether solution was cooled to -20 'C and the product was obtained as a black
microcrystalline solid; yield 152 mg (78%).
1H
NMR (C6 D 6 ) 6 3.74 (TMS), -17.05
(NCH 2 CH 2 N), -30.77 (NCH 2 CH 2 N). UV-visible(Toluene) X= 542 nm, E = 17,872 M- 1 cm- 1 .
Anal. Calcd. for C3 0H 78 N10Si 6 Mo2: C, 38.36; H, 8.37; N, 14.91. Found: C, 38.09; H, 8.45; N,
14.48.
[(Me 3 SiNCH 2 CH2) 3 N]MoN2SiMe3 (6). Method 1. { [N3 N]MoN 2
2 Mg(THF) 2
(75 mg, 0.07 mmol) was dissolved in 5 mL THF. TMSC1 (20 gL, 0.16 mmol) was added by
syringe. The color of reaction mixture immediately lightened to yellow. The solution was stirred
for 2 h. THF was removed and the residue extracted into pentane and filtered through Celite. The
pentane solution was reduced and cooled to -30 'C to give a yellow solid; yield 60 mg (77%).
Method 2. [N 3 N]MoCl (100 mg, 0.20 mmol) was dissolved in 5 mL THF.
Magnesium
powder and Me 3 SiCl (80 mg, 0.74 mmol) were added. The solution was stirred and after 5 h the
solution was yellow. After 20 h the THF was removed and the residue extracted into pentane and
filtered through Celite. The pentane solution was reduced and cooled to -30 'C give a yellow solid;
yield 100 mg (88 %). 1H NMR(C 6 D6 ) 8 3.38 (t, 6H, NCH 2 CH 2 N), 2.10 (t, 6H, NCH 2 CH 2 N),
0.49 (s, 27H, NSiMe 3 ), 0.49 (s, 9H N2 SiMe 3 ).
13 C{ 1H}
NMR(C 6 D6 ) 8 54.1 (NCH 2 CH 2 N),
52.1 (NCH 2 CH2 N), 4.1 (NSiMe3), 4.0 (N2SiMe3). IR(THF, cm- 1) 1714 (N=N). IR(Nujol, cmnr
1) 1712 (N=N). Anal. Calcd. for C1 8H 48 N 6 Si 4 Mo: C, 38.82; H, 8.69; N, 15.09. Found: C,
38.86; H, 8.73; N, 15.02.
References begin on page 65
60
Chapter1
6- 15 N2. This compound was prepared in a manner analogous to method 1 used to prepare
6, except 1- 15 N 2 was used.
15 N
NMR(C 6 D6 ) 6 356.9 (JNN = 12), 238.1 (JNN = 12). IR(THF,
cm- 1) 1654 (N=N).
{ [N(CH 2 CH 2 NSiMe 3 )2 (CH 2 CH 2 NCH 3 )]MoN 2 (CH 3 ) 2 }+OTf-(THF)o.s (7).
[N3 N]MoN 2 SiMe3 (500 mg, 0.90 mmol) was dissolved in 30 mL of toluene and cooled to -20 'C.
Methyl triflate (408 pL, 3.60 mmol) was dissolved in 15 mL of toluene and cooled to -20 'C. The
methyl triflate solution was added dropwise to the stirred solution of [N3N]MoN2SiMe 3 . After 18
h the toluene was removed and the residue was washed with 7 mL of pentane to remove any
unreacted [N3 N]MoN 2 SiMe 3 (50 mg, 0.09 mmol). The brown-red solid was dissolved in
minimum THF and filtered. Ether was added and the solution stored at -20 'C to give 102 mg of
orange crystals (20%). A second recrystallization from THF/pentane was performed.
1H
NMR(THF-d 8 ) 8 3.96-3.79 (m, NCH 2 CH 2 , 6H), 3.73 (s, NCH 3 , 3H), 3.72 (s, N(CH 3 )2 , 6H),
3.43-3.26 (m, NCH 2 CH 2 , THF, 8H), 1.58 (m, THF), 0.27 (s, NTMS, 18H).
13 C
NMR(THF-
ds) 8 71.50 (t, THF, 1JCH = 138), 64.10 (t, NCH 2 CH 2 N, 1JCH = 138), 56.79 (t, NCH 2 CH 2 N,
1 JCH
= 140), 55.50 (q, NCH 3 , 1 JCH = 136), 54.26 (t, NCH 2 CH 2 N, 1JCH = 140), 53.09 (t,
NCH 2 CH 2 N, 1JCH = 140), 46.15 (q, N(CH 3 ) 2 , 1JCH = 141), 27.82 (t, THF, 1JCH = 127), 2.73
(q, NTMS,
1 JCH
= 119).
19 F
NMR(C 6 D 6 )8
-77.3 (s, CF 3 SO 3 ).
Anal. Calcd. for
C18H4 3 F3 Si 2 N6MoO 3 .5 S: C, 33.74; H, 6.76; N, 13.12. Found: C, 33.61; H, 6.83; N, 12.93
7- 15 N2 . This complex was synthesized in an analogous manner to 7 except 6- 15 N2 was
used.
15 N
NMR(THF-ds) 8 374.81 (d, 1JNN = 12), 157.15 (d, 1JNN = 12).
{[N(CH2CH2NSiMe3)2(CH
2 CH2NMe 2 )]MoN 2 TMS}+OTf
(8).
Having
isolated 7 from the reaction mixture, pentane was added to the mother liquor which was then
cooled to -20 OC. The red solid obtained was subjected to a second crystallization from
THF/pentane to give the product as a pink/red solid.
1H
NMR(THF-d 8 ) 8 4.17 (m, 2H,
NCH 2 CH 2 N), 3.93 (m, 2H, NCH 2 CH 2 N), 3.35 (m, 2H, NCH2CH 2 N), 3.24 (m, 4H,
NCH 2 CH 2 N), 2.96 (m, 2H, NCH 2 CH 2 N), 2.76 (s, 6H, N(CH 3 )2 ), 0.34 (s, 27H, NTMS).
13 C{
1H} NMR(THF-d 8 ) 8 61.42 (NCH 2 CH 2 N), 54.79 (NCH 2 CH 2 N), 54.40 (NCH CH N),
2
2
References begin on page 65
Chapter1
51.38 (NCH 2 CH2N), 48.26 (N(CH3)2), 2.99 (NTMS), 2.57 (NTMS). IR(Nujol) cm- 1 1724
(N=N). Anal. Calcd. for C1 8 H4 5 F 3 Si 3 N 6 MoO3S: C, 32.62; H, 6.84; N, 12.68. Found: C,
32.59; H, 6.93; N, 12.56.
8- 15 N2 . This complex was synthesized in an analogous manner to 8 except 6- 15 N2 was
used. IR(Nujol) cm- 1 1668 (N=N).
1JNN
15 N
NMR(THF-d 8 ) 8 361.50 (d, 1 JNN = 13), 244.34 (d,
= 13).
{[N(CH2CH2NSiMe
3 )2(CH2CH 2 NMe 2 )]MoN 2 TMS(CH3)
(9).
{[N(CH 2 CH 2 NSiMe3 )2 (CH 2 CH 2 NMe 2 )]MoN2TMS }+OTf (200 mg, 0.302 mmol) was dissolved
in 5 mL of diethyl ether and cooled to -20 oC. 3.14 M MeMgCl (96 gL) was added dropwise to
the stirred solution. After 20 min, the solvent was removed in vacuo and the residue extracted with
pentane. Following filtration through Celite, the pentane was removed in vacuo to give the product
as an orange/red solid; yield 153 mg (96%).
1H
NMR(C 6 D6 ) 8 3.54 (t, 1H, CH 2 ), 3.49 (t, 1H,
CH 2 ), 3.35 (t, 1H, CH 2 ), 3.31 (t, 1H, CH 2 ), 2.97 (t, 2H, CH2), 2.43 (t, 4H, CH2), 2.03 (t,
2H, CH 2 ), 1.90 (s, 6H, NCH 3 ), 0.74 (s, 3H, MoCH 3 ), 0.53 (s, 18H, NTMS), 0.50 (s, 9H,
NTMS).
13 C
NMR(C 6 D6 ) 8 53.84 (t, NCH2 CH 2 N), 52.86 (t, JCH = 138, NCH 2 CH 2 N), 52.08
(t, JCH = 134, NCH 2 CH2N), 51.65 (t, JCH = 138, NCH 2 CH 2 N), 45.93 (q, JCH = 135,
CH 3 NNCH 3 ), 23.92 (q, JCH = 121, MoCH3), 3.90 (q, JCH = 118, NTMS), 3.56 (q, JCH = 118,
NTMS). IR(Nujol) cnrm- 1 1640 (N=N). Anal. Calcd. for C18H48Si3N6Mo: C, 40.88; H, 9.15; N,
15.89. Found: C, 40.72; H, 8.97; N, 15.55.
9- 15 N2. This complex was synthesized in an analogous manner to 9 except 8- 15 N2 was
used.
15 N
NMR(C 6 D6 ) 8 374.62 (d, 1JNN = 15), 239.46 (d, 1JNN = 15). IR(Nujol) cm- 1 1577
(N=N).
{ [N(CH 2 CH
2 NSiMe 3 )2 (CH2CH2N
C H 3 )]Mo(CH 3 )N 2 (CH
3) 2 }
(10).
{[N(CH 2 CH 2 NSiMe 3 )2 (CH2CH 2 NCH 3 )]MoN2(CH 3 )2 }+OTf-(THF)0. 5 (178 mg, 0.294 mmol)
was dissolved in 7 mL THF and cooled to -20 'C. 98 gL (1.05 eqs) of 3.14 M MeMgCl in THF
was diluted to 3 mL with THF and cooled to -20 OC. Upon addition of MeMgCl to the stirred
solution of {[N(CH 2 CH 2NSiMe 3 )2 (CH 2 CH2NCH 3 )]MoN 2 (CH 3 )2 }+OTf-(THF)0. 5 , the color
References begin on page 65
ChapterI
immediately changed from orange to blood-red. After 20 min the solvent was removed in vacuo
and the residue extracted with pentane. Following filtration through Celite, the pentane was
removed in vacuo to give the product as a red film; yield 125 mg (90%).
1H
NMR(C 6 D6 ) 8 3.79
- 3.72 (m, 1H, CH 2 ), 3.66 - 3.58 (m, 2H, CH 2 ), 3.56-3.52 (m, 1H, CH 2 ), 3.51
(s, 3H,
NCH 3 ), 3.30 - 3.24 (m., 1H, CH 2 ), 3.02 - 2.95 (m, 1H, CH 2 ), 2.91 - 2.89 (m, 2H, CH 2 ), 2.84
(s, 6H, N(CH 3 )2 ), 2.61 - 2.55 (m, 1H, CH 2 ), 2.52 - 2.46 (m, 1H, CH 2 ), 2.38 - 2.33 (m, 1H,
CH 2 ), 2.20 - 2.14 (m, 1H, CH 2 ), 0.38 (s, 9H, NTMS), 0.27 (s, 9H, NTMS), 0.18 (s, 3H,
MoCH3).
13 C
NMR(C 6 D6 ) 8 67.19 (t, CH 2 , JCH = 129), 66.21 (t, CH 2 , JCH = 133), 64.76 (t,
CH2, JCH = 136), 60.91 (t, CH 2 , JCH = 136), 56.29 (t, CH 2 , JCH = 131), 54.00 (q, NCH 3 , JCH
= 136), 52.94 (t, CH 2 , JCH = 131), 44.30 (q, N(CH 3 )2 , JCH = 136), 17.79 (q, MoCH 3 , JCH =
124), 3.01 (q, NTMS, JCH = 118), 2.54 (q, NTMS, JCH = 118). Due to the thermal instability of
this compound a sample for elemental analysis was not obtained.
10-
15 N
2.
This complex was synthesized in an analogous manner to 10.
15 N
NMR(THF-ds) 8 354.85 (d, 1JNN = 12), 141.97 (d, 1JNN = 12).
[ N (C H
2
C H 2N Si Me 3)2( C H
2
C H
2
N C H 3)]MoN
(11).
{[N(CH 2 CH 2 NSiMe 3 )2 (CH 2CH 2 NCH3H]Mo(CH 3 )N 2 (CH 3 ) 2 (115 mg, 0.24 mmol) was
dissolved in 1.5 mL of C6 D6 and placed in a glass bomb along with a stirring bar. The bomb was
sealed, removed from the dry box and the contents heated to 84 'C for 15 h. The volatiles were
vacuum-transferred into an NMR tube which was then sealed. The bomb was returned to the dry
box and the residue extracted with pentane and filtered. Following filtration the volume was
reduced in vacuo and the solution chilled to -20 'C to give the product as yellow needles; yield 30
mg (30%).
1H
NMR(C 6 D 6 ) 8 4.08 (s, 3H, NCH 3 ), 3.25 (t, 4H, NCH 2 CH 2 N), 2.78 (t, 2H,
NCH 2 CH 2 N), 2.20-2.00 (m, 6H, NCH 2 CH 2 N), 0.59 (s, 18H, NTMS).
13 C
NMR(C 6 D6 ) 8
60.38 (q, NCH 3 ), 59.73 (t, NCH 2 CH 2 N), 52.77 (t, NCH 2 CH 2 N), 51.17 (t, NCH 2 CH 2 N),
49.81 (t, NCH 2 CH 2 N), 3.18 (q, NTMS).
IR(Nujol) cm - 1 1002 (Mo-N).
Anal. Calcd. for
C 13 H33 Si 2 N5 Mo: C, 37.94; H, 8.08; N, 17.02. Found: C, 37.96; H, 7.51; N, 16.69
References begin on page 65
Chapter1
11-
15 N
NMR(C6D6)
2.
This complex was synthesized in an analogous manner to 11.
15 N
866.08 (s). IR(Nujol) cm- 1 977 (Mo-N).
Thermolysis of 10 in THF-d 8 ; identification of methane and dimethylamine.
{ [N(CH 2 CH 2 NSiMe 3 )2 (CH 2 CH 2 NCH 3 )]Mo(CH 3 )N 2 (CH 3 ) 2 (38 mg, 0.08 mmol) was
dissolved in 0.5 mL of THF-d8 and placed in a teflon stoppered NMR tube. Cyclohexane (4.4
pL, 0.04 mmol) was added as an internal standard and the tube was sealed. The solution was
heated at 76 oC for 12 h.
1H
NMR(THF-d8 ) 8 3.97 (s, 3H, NMe), 3.56 (m, 4H, NCH 2 CH 2 N),
3.29 (t, 2H, NCH 2 CH 2 N), 3.26 (d, unassigned), 2.81 (t, 2H, NCH 2 CH 2 N), 2.65 (m, 4H,
NCH 2 CH 2 N), 2.42 (s, unassigned), 2.31 (d, (CH 3 ) 2 NH), 1.44 (s, cyclohexane), 0.32 (s,
unassigned), 0.29 (s, 18H, NTMS), 0.20 (s, unassigned), 0.19 (s, CH4 ), 0.13 (s, unassigned),
0.09 (s, unassigned), 0.07 (s, unassigned), 0.06 (s, unassigned), 0.01 (s, unassigned).
13 C
NMR(THF-ds) 8 60.3 (NCH 3 ), 60.1 (NCH2 CH 2N), 58.6 (unassigned), 56.2 (unassigned), 55.6
(unassigned), 55.5 (unassigned), 55.3 (unassigned), 53.7 (unassigned), 53.3 (NCH 2 CH 2 N),
52.3 (unassigned), 51.5 (NCH 2 CH 2 N), 50.4 (NCH 2 CH 2 N), 49.5 (unassigned), 44.7
(unassigned), 44.2 (unassigned), 39.3 ((CH 3)2 NH), 38.2 (unassigned), 27.8 (cyclohexane), 3.1
(unassigned), 2.73 (TMS), 2.5 (unassigned), 2.0 (unassigned), 1.3 (unassigned), -1.2 (CH 4 ).
[N 3 N]MoEN
(12). [N 3 N]MoCl (106 mg, 0.22 mmol) was dissolved in 10 mL
toluene and placed in a bomb. TMSN 3 (120 gL, 0.90 mmol) was added by syringe and the bomb
was sealed. The solution was heated at 90 °C for 24 h during which time the color of the reaction
mixture changed to brown/yellow. The solvent was removed and the residue was extracted into
pentane and filtered. The filtrate was reduced in volume and cooled to -30 OC to give the product as
a yellow crystalline compound; yield 89 mg (88%).
CH 2 ), 0.56 (s, SiMe 3 ).
13 C{ 1 H}
1H
NMR(C 6 D6 ) 8 3.23 (t, CH 2 ), 2.14 (t,
NMR(C 6 D6 )8 52.2 (CH 2 ), 51.6 (CH 2 ), 3.1 (SiMe 3 ); IR
(Nujol) cm- 1 1001 (Mo-N). Anal. Calcd for C15H 39 N 5 Si 3 Mo: C, 38.36; H, 8.37; N, 14.91.
Found: C, 37.99; H, 8.17; N, 14.65.
{[N 3 N]Mo=NMe}OTf (13).
[N3 N]Mo=-N (104 mg, 0.22 mmol) was dissolved in 3
mL toluene and MeOTf (30 pgL, 0.27 mmol) was added by syringe. The reaction mixture
References begin on page 65
Chapter1
immediately deepened in color and a yellow solid precipitated. After 1 h the solvent was removed
in vacuo and the resulting solid was dissolved in the minimum volume of THF. The solution was
cooled to -30 °C to give the product as yellow needles; yield 129 mg (93%).
1H
NMR (CD2 C12 ) 6
13 C{ 1 H}
4.45 (s, 3, NCH 3 ), 4.00 (t, 6, CH2), 3.24 (t, 6, CH 2 ), 0.30 (s, 27, SiMe3).
(CD
2 C1 2 )
NMR
Anal. Calcd for
8 57.0 (NCH 3 ), 56.2 (CH 2 ), 54.5 (CH 2 ), 3.2 (TMS).
C 17 H4 2 N5 Si 3F 3 SO 3 Mo: C, 32.22; H, 6.68; N, 11.05. Found: C, 32.07; H, 6.61; N, 11.17.
{[N 3 N]Mo=NSiMe3}OTf (14). [N3 N]Mo=-N (75 mg, 0.16 mmol) was dissolved in
3 mL toluene and TMSOTf (40 jtL, 0.21 mmol) was added by syringe. The reaction mixture
immediately deepened in color and a yellow solid precipitated. After 2 h the solvent was removed
in vacuo and the resulting solid was dissolved in the minimum volume of THF. The solution was
1H
NMR(CD 2 C12) 6
13 C{IH}
NMR (CD2 C12 )
cooled to -30 OC to give the product as yellow needles; yield 92 mg (83%).
3.85 (t, CH2), 3.11 (t, CH2), 0.58 (s, 9, SiMe3), 0.33 (s, 27, SiMe3).
8 58.2 (CH 2 ), 57.1 (CH2), 3.9 (SiMe3), 2.4 (SiMe3). Anal. Calcd for C 19 H4 8 N5 Si4F3SO3Mo:
C, 32.98; H, 6.99; N, 10.12. Found: C, 32.84; H, 6.47; N, 9.58.
REFERENCES
(1) Bazhenova, T. A.; Shilov, A. E. Coord. Chem. Rev. 1995, 144, 69.
(2) Eady, R. R.; Leigh, G. J. J. Chem. Soc., Dalton Trans. 1994, 2739.
(3) Eady, R. R. Chem. Rev. 1996, 96, 3013.
(4) Chan, M. K.; Kim, J. S.; Rees, D. C. Science 1993, 260, 792.
(5) Allen, A. D.; Senoff, C. V. J. Chem. Soc., Chem. Commun. 1965, 621.
(6) Hidai, M.; Mizobe, Y. Chem. Rev. 1995, 95, 1115.
(7) Chatt, J.; Dilworth, J. R.; Richards, R. L. Chem. Rev. 1978, 78, 589.
(8) Leigh, G. J. Acc. Chem. Res. 1992, 25, 177.
(9) Hidai, M.; Ishii, Y. Bull. Chem. Soc. Jpn. 1996, 69, 819.
(10) Schrock, R. R.; Glassman, T. E.; Vale, M. G. J. Am. Chem. Soc. 1991, 113, 725.
Chapter1
(11)
Schrock, R. R.; Glassman, T. E.; Vale, M. G.; Kol, M. J. Am. Chem. Soc. 1993, 115,
1760.
(12) Vale, M. G.; Schrock, R. R. Inorg. Chem. 1993, 32, 2767.
(13) Laplaza, C. E.; Cummins, C. C. Science 1995, 268, 861.
(14) Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.; Kim, E.; Cummins, C. C.;
George, G. N.; Pickering, I. J. J. Am. Chem. Soc. 1996, 118, 8623.
(15) Fryzuk, M. D.; Love, J. B.; Rettig, S. J.; Young, V. G. Science 1997, 275, 1445.
(16) Verkade, J. G. Acc. Chem. Res. 1993, 26, 483.
(17) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9.
(18) Cummins, C. C.; Schrock, R. R.; Davis, W. M. Angew. Chem. 1993, 115, 758.
(19)
Moisch-Zanetti, N. C.; Schrock, R. R.; Davis, W. M.; Wanninger, K.; Seidel, S. W.;
O'Donoghue, M. B. J. Am. Chem. Soc. 1997, 119, 11037.
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W. M. Organometallics1997, 16, 5195.
(21) Schrock, R. R.; Seidel, S. W.; Misch-Zanetti, N. C.; Shih, K. -Y.; O'Donoghue, M. B.;
Davis, W. M.; Reiff, W. M. J. Am. Chem. Soc. 1997, 119, 11876.
(22) Kol, M.; Schrock, R. R.; Kempe, R.; Davis, W. M. J. Am. Chem. Soc. 1994, 116, 4382.
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(26) K. -Y. Shih, unpublished observations.
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Engl. 1976, 15, 612.
(29) D. A. Dobbs, unpublished observations.
(30) Seidel, S. W., Ph.D. Thesis, MIT, 1998.
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Chapter1
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1980, 786.
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2694.
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5740.
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1988, 2935.
(38) Plass, W.; Verkade, J. G. J. Am. Chem. Soc. 1992, 114, 2275.
(39) Nugent, W. A.; Mayer, J. M. Metal-LigandMultiple Bonds; Wiley: New York, 1988.
(40) G. E. Greco, unpublished observations.
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Organometallics1996, 15, 1518.
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CHAPTER 2
Heterometallic Dinitrogen Complexes Containing the
{[N3N]Mo(N 2 ) }- Ligand
A portion of the material covered in this chapter has appeared in print:
O'Donoghue, M. B., Zanetti, N. C., Davis, W. M., Schrock, R. R. J. Am. Chem. Soc.
1997, 119,2753.
Chapter2
INTRODUCTION
Historically, transition metal dinitrogen complexes can be divided into two broad categories
namely monometallic and bimetallic complexes, and conceptually it is possible to reduce dinitrogen
to ammonia in either type of system. Numerous examples of monometallicl,' 2 and homobimetallic 3
dinitrogen complexes have been described in the literature, yet examples of heterometallic
dinitrogen complexes are comparatively rare. In fact, only four such complexes have been
structurally characterized, namely dinuclear [(PMe 2 Ph) 4 C1Re(N 2 )MoC1 4 (OMe)],
[WI(PMe 2 Ph) 3 (py)(N2)ZrCp2Cl]
5
4
and [Cp*Me 3 Mo(N 2 )WCp'Me 3 ], 6 and trinuclear
7
[MoC14 (N2 )ReCl(PMe 2 Ph)4 }21.
The paucity of heterometallic dinitrogen complexes prompted us to explore the synthesis of
such complexes and the results of our efforts are detailed in this chapter and are summarized in
Scheme 2.1. Recall that {[N3 N]Mo-(N=N) }2 Mg(THF) 2 is isolated in high yield from the
reduction of [N3 N]MoCl under dinitrogen (Chapter 1). While initial efforts focused on the
derivatization of dinitrogen at a single metal center, we realized that the {[N3 N]Mo-(N=N)}fragment might be employed as a ligand in the synthesis of heterometallic dinitrogen complexes.
In view of the crystal structure of nitrogenase 8 which confirms the presence of both iron and
molybdenum in the active site, the synthesis of an iron/molybdenum dinitrogen complex was set as
the initial goal in order to demonstrate how dinitrogen could be bound between these biologically
relevant metals.
The syntheses and structural characterization of iron/molybdenum,
vanadium/molybdenum and zirconium/molybdenum dinitrogen complexes containing the
1
{[N3 N]Mo(N 2 ) }- ligand are presented. As several of the resulting complexes are paramagnetic H
NMR spectroscopy has been of limited use as a method of characterization. X-ray crystallography
has been used extensively and three structural studies are reported including that of { [N3 N]MoN=N}3Fe, the first example of a structurally characterized iron/molybdenum dinitrogen complex.
The oxidation state of iron in this complex and in {[N 3 N]Mo-N=N} 2 Fe(DMPE) has been
established by M6ssbauer spectroscopy.
References begin on page 102
Chapter2
Scheme 2.1. Synthesis of heterometallic dinitrogen complexes.
TMS
/
N-oN
N - Mo ,jjNN
's
N
TMS
TMS
N
I
T MS
TMS
//
N
N
\
Mo
"
N'-TMS
TMS
DMPE
Fe
TMS
N
NN
TMS mII-;N-N;
TMS 0
..
FeC12
{[N 3N]Mo(N 2 ) }2 Mg(THF) 2
ZrC14(THF)2
VC13 (THF)3
VC14 (DME)
[N 3N]Mo(N 2 )
iv
N
CI
N
V
TMS
N
N
TMS F
)NZ
/
S
References begin on page 102
TMSN
TMS
Chapter2
RESULTS AND DISCUSSION
Iron/Molybdenum Dinitrogen Complexes
Addition of FeC12 to a 10:2:1 Et 2 0/THF/toluene solution of {[N3N]Mo(N 2 )12Mg(THF) 2
results in a darkening of the solution over the course of 15 min. {[N3 N]Mo(N 2 )13 Fe (1) can be
isolated in 38% yield from the pentane extract of the crude reaction product as plum-colored,
paramagnetic crystals. Since a black magnetic solid, presumed to be iron, is formed during the
course of the reaction and is observed clinging to the stir bar, the ideal stoichiometry for the
reaction would be that shown in equation 1. The 1H NMR spectrum of 1 in C6 D6 exhibits three
broad, shifted resonances at 9.25, -9.71 and -64.0 ppm consistent with a species in which the
[N3 N]Mo portion of the molecule is C3-symmetric but the spectrum provides no information as to
the molecular structure of 1. An IR spectrum of 1 in Nujol shows primarily an absorption at 1703
cm- 1 , although weaker absorptions are present between 1703 and 1600 cm- 1 suggesting that
dinitrogen is present and acting as a diazenido (2-) ligand. The UV-visible spectrum of 1 in
pentane has an intense absorption at 516 nm (E = 22,800 M- 1 cm-l) that shifts to 476 nm upon
addition of THF (see below). Satisfactory elemental analyses of 1 have not been obtained due to
the presence of trace amounts of [N3 N]Mo(N 2 ) in the samples.
{[N 3N]Mo-N=N }2 Mg(THF) 2 + FeC12
Et 20/THF/toluene
t
-20 C - r.t.
2/3 {[N 3N]Mo-N=N} 3Fe
1
(1)
+ MgCl 2 + 1/3 Feo
The molecular structure of 1 was elucidated by an X-ray crystallographic study. Crystals
of 1 suitable for X-ray analysis were grown from saturated pentane solutions at -20 °C; a quarter
of a molecule of pentane was found in the unit cell. Crystallographic data and collection and
refinement parameters are given in Table 2.1. A view of the molecular structure of 1 along with
the atom-labeling scheme is shown in Figure 2.1, while pertinent bond lengths and bond angles are
listed in Table 2.2.
Table 2.3 summarizes selected metrical parameters for all of the
References begin on page 102
Chapter2
crystallographically-characterized complexes reported in this chapter. Although the structure is not
of high quality it does shed light on the remarkable connectivity of 1 which is a rare example of a
complex with iron in a trigonal planar environment. The [N3 N]Mo(N 2 ) unit can be viewed as a
ligand with the bulky TMS groups of the triamidoamine precluding the attainment of higher
coordination numbers. The three Mo-N-N linkages are essentially linear as are two of the Fe-N-N
linkages. However, one of the Mo-N-N-Fe linkages is significantly bent at the nitrogen bound to
iron (Fe-N(2)-N(1) = 156(2)'). The deviation from linearity of Fe-N(2)-N(1) is perhaps a
consequence of steric crowding created by the [N3N] 3 - ligand. In previous work,9 it has been
found that the twisting of a given TMS group out of the Nax-M-Neq plane and the resulting
decrease in the Nax-M-Neq-Si dihedral angle are useful measures of the degree of steric strain in the
pocket of [N3 N] complexes. In 1 the dihedral angle defined by N(14)-Mo(l)-N(13)-Si(13) is
found to be 142.50, indicative of considerable steric pressure arising from three [N3 N]Mo(N 2 )
units lying in the trigonal plane. However, all other Nax-M-Neq-Si dihedral angles are close to
180'. In view of the relatively large errors we cannot say that distances within the [N3 N]Mo(N 2 )
units are statistically different. Nevertheless, the N-N bond distances suggest reduction of the
dinitrogen ligands in 1 compared with free dinitrogen (1.098 O).10
Three coordinate iron complexes have been known for some time and can be grouped into
three broad classes, namely, dimeric Fe(II) complexes such as {Fe[O-(2,4,6-tBu 3C 6 H2 )2] }211 and
{Fe(NPh 2 )2 12 12 which contain terminal and bridging alkoxy or amide ligands, monomeric Fe(II)
complexes such as Fe[N(SiMe 3 )22(THF)1 2 and {Fe[N(SiMe 3 )2 13} - 13 and monomeric Fe(III)
complexes of which only two other examples are known namely, Fe(NRAr)3 14 (R =
C(CD 3 )2 CH 3 , Ar = 3,5-C 6 H3 Me 2 ) and Fe[N(SiMe 3 )2] 3 .15 1 is unique among these complexes
for several reasons.
Firstly, crystallographically-characterized, heterometallic complexes
containing bridging dinitrogen are rare and to our knowledge 1 is the only reported example of a
structurally characterized iron-molybdenum dinitrogen complex, a type of species that perhaps is
especially relevant in view of the structure of Fe/Mo nitrogenase in one resting state. 8
References begin on page 102
Chapter2
Table 2.1. Crystallographic data, collection parameters and refinement parameters for
{[N3 N]Mo-N=N} 3Fe (1) and {[N3 N]Mo-N=N }2 VCI(THF) (4).
Empirical Formula
C46 .25 H1 18.5 FeMo 3 N 8 Si 9
C39 H9 8.5 0 C1Mo 2 N 12 02. 25 Si 6 V
Formula Weight
1521.05
1218.61
Diffractometer
SMART/CCD
SMART/CCD
Crystal Dimensions (mm)
0.14 x 0.14 x 0.12
na
Crystal System
Triclinic
Monoclinic
Space Group
P1
P2 1/n
a (A)
10.4926(2)
14.16110(10)
b (A)
14.3300(10)
21.61220(10)
26.8875(6)
21.1463(3)
97.2850(10)
90
f(0)
93.2670(10)
98.6770(10)
y (0)
90.163(2)
90
V (A3), Z
4001.93(12), 2
6397.81(11), 4
Dcale (Mg/m 3)
1.262
1.265
Absorption coefficient (mm-l)
0.811
0.723
F000
1595
2570
Temperature (K)
188(2)
183(2)
E range for data collection (0)
1.53 to 20.00
1.39 to 23.29
Reflections collected
11831
25386
Unique Reflections
7337
9183
R
0.1345
0.0660
Rw
0.1896
0.1080
GoF
1.280
-~---
1.085
c(0)
References begin on page 102
Chapter2
Table 2.2. Selected bond lengths and bond angles for { [N3N]Mo(N 2 )13 Fe (1).
Bond Lengths (A)
Fe-N(2)
1.86(2)
Fe-N(4)
1.84(2)
Fe-N(6)
1.82(2)
Mo(1)-N(1)
1.86(2)
Mo(2)-N(3)
1.81(2)
Mo(3)-N(5)
1.82(2)
N(1)-N(2)
1.20(3)
N(3)-N(4)
1.25(2)
N(5)-N(6)
1.27(2)
Mo(1)-N(11)
1.97(2)
Mo(2)-N(23) 2.03(2)
Mo(3)-N(32) 2.00(2)
Mo(1)-N(14) 2.24(2)
Mo(2)-N(24) 2.26(2)
Mo(3)-N(34) 2.24(2)
Bond Angles (deg)
Mo(1)-N(1)-N(2)
174(2)
Mo(2)-N(3)-N(4)
175(2)
Mo(3)-N(5)-N(6)
179(2)
Fe-N(2)-N(1)
156(2)
Fe-N(4)-N(3)
175(2)
Fe-N(6)-N(5)
176(2)
Mo(1)-N(11)-Si(11)
127.4(12)
Mo(2)-N(23)-Si(23)
123.7(10)
N(2)-Fe-N(4)
114.0(9)
N(2)-Fe-N(6)
119.2(10)
N(4)-Fe-N(6)
126.8(9)
Dihedral Angles (deg)a
N(14)-Mo(1)-N(11 )-Si(11)
170.4
N(14)-Mo(1)-N(13)-Si(13)
142.5
N(14)-Mo(1)-N(12)-Si(12)
-164.7
N(24)-Mo(2)-N(23)-Si(23)
176.3
N(34)-Mo(3)-N(32)-Si(32)
-177.9
aObtained from a Chem-3D Drawing
References begin on page 102
~
Il-LI -
~I-
i
--------
1--
I L_ L--
C-
~.-^- s-
I~
Chapter2
Figure 2.1. A view of the structure of { [N3 N]Mo-N=N } 3 Fe (1) with the trigonal plane lying in
the plane of the paper.
Secondly, the three ligands coordinated to iron are all derived from dinitrogen and finally, the
dinitrogen-containing ligands can exist in both anionic and neutral forms (see Chapter 1).
In light of the extraordinary molecular geometry of 1, magnetic susceptibility and
M6ssbauer studies were embarked upon with a view to establishing the spin state and oxidation
state of iron in 1, information that is not immediately apparent from the X-ray diffraction data.
References begin on page 102
Chapter2
Since the [N3N]Mo(N2) ligand is stable as the terminal dinitrogen complex, [N3 N]Mo(N 2 ) and the
diazenido complex, {[N3 N]Mo(N2)} 2 Mg(THF) 2 , 1 could be formulated, at the extremes, as an
Fe(0) complex containing three neutral [N3 N]Mo(N 2 ) ligands or as an Fe(lI) complex containing
three anionic {[N3 N]Mo-N=N }- ligands. Alternatively, 1 may exist as an Fe(II) complex with one
neutral and two anionic ligands. The formulation of 1 as an Fe(0) complex can be ruled out on the
basis of the N-N bond lengths. Although the errors are large, N-N bond lengths of 1.25(2) and
1.27(2)
A are more consistent with a diazenido
N(1) and the N(1)-N(2) bond length of 1.20(3)
complex. The significant bending of the Fe-N(2)-
A suggest that the three [N3 N]Mo(N 2 ) ligands
are
inequivalent and so 1 might be viewed as an Fe(II) complex. However, magnetic susceptibility
and M6ssbauer studies unequivocally demonstrate that 1 is best formulated in the solid state as an
Fe(lI) complex (see below).
Table 2.3. Selected metrical parameters for heterometallic dinitrogen complexes.
L-M-L (deg)
Complex
N-N (A)
Mo-N (A)
M-N (A)
N-M-N (deg)
Mo/Fe (1)
1.20(3)
1.86(2)
1.86(2)
119.2(10)
1.25(2)
1.81(2)
1.84(2)
114.0(9)
1.27(2)
1.82(2)
1.82(2)
126.8(9)
1.217(7)
1.827(6)
1.860(6)
119.8(3)
96.5(2)
1.221(7)
1.836(6)
1.864(4)
1.249(8)
1.796(6)
1.974(6)
114.6(2)
107.14(9)
1.245(8)
1.797(6)
1.978(6)
MoNV (4)
Mo/Zr (6)
SQUID 16 magnetic susceptibility data for solid 1 is plotted versus temperature in Figure
2.2 and can be fit to the Curie-Weiss law (x = g 2 /8(T-0)) over the temperature range 5-300 K to
yield g. = 6.02(3) gB, 0 = 0.74(5) K. These data are unremarkable other than that the value for g.
References begin on page 102
Chapter2
is close to the spin-only value for a system containing five unpaired electrons (5.92 ttB) and is
consistent with the formulation of 1 as a high-spin Fe(III) complex.
Figure 2.2. Plot of Xm (corrected for diamagnetism using Pascal's constants) versus T for
{[N3 N]Mo-N=N }3 Fe, (1).
1.2
1
0.8
Xim
0.6
0.4
0.2
0
0
50
100
150
200
250
300
350
T (K)
Further evidence confirming the identity of 1 as a high-spin Fe(III) complex was obtained
from Mossbauer spectroscopic studies (carried out by Professor William Reiff). Figure 2.3 shows
the M6issbauer spectrum of 1 obtained at 77 K. It consists of a quadrupole doublet with
quadrupole splitting of 3.15 mm/sec and an isomer shift of 0.65 mm/sec relative to natural iron
foil. The high energy peak is broadened, a common feature in high-spin Fe(III) complexes 17
which arises from paramagnetic relaxation phenomena. 18 The quadrupole splitting arises from the
presence of an electric field gradient at the M6ssbauer nucleus and the magnitude of the quadrupole
splitting reflects the asymmetry of the electron density around the nucleus. 19 In 1, the absence of
axial ligands gives rise to an unusually large electric field gradient and hence the large quadrupole
splitting. The magnitude of the isomer shift is reasonable for a ferric complex and is also
References begin on page 102
400
1.00.
2.00
77K
I
-4
I
I
I
I
I
I
2
0
-1
-2
-3
VELOCITY (mm/sec) RELATIVE TO NATURAL IRON FOIL
Figure 2.3. M6ssbauer spectrum of { [N3N]Mo-N=N) 3 Fe (1) at 77 K.
Chapter2
consistent with the low coordination number. 19 Fe[N(SiMe3)213 is the only other example of an
Fe(III) complex containing trigonal planar iron which has been studied in detail by X-ray
crystallography, 15 magnetic susceptibility measurements 20 and M6ssbauer spectroscopy. 17
Unlike 1, resonances were not found in the 1 H NMR spectrum of Fe[N(SiMe 3 )213, presumably
due to the proximity of the TMS groups to the paramagnetic center. 2 1 The magnetic moment of
Fe[N(SiMe 3 )2 13 obtained between 98-298 K is 5.94 gB which like that of 1 is close to the spinonly moment for a system with five unpaired spins. Finally, the MSssbauer spectrum of
Fe[N(SiMe 3 )2 13 at 77 K is strikingly similar to that of 1, consisting of an asymmetric quadrupole
doublet with quadrupole splitting of 5.12 mm/sec. The M6ssbauer spectral parameters of 1 and
Fe[N(SiMe3)213 allow a qualitative comparison of the bonding in these complexes to be made. o
donation by the ligands increases the total electron density at the nucleus and nr acceptance by the
ligands decreases shielding of the s electron density, both of which have the effect of decreasing
the isomer shift. Since the {[N3 N]Mo-N=N)- ligand is expected to be a weak I acceptor, the
smaller isomer shift of Fe[N(SiMe 3 )2 13 (0.30 mm/sec) reflects the better a donating ability of the
[N(SiMe 3 )2]- ligand compared to the {[N3 N]Mo-N=N}- ligand. The larger quadrupole splitting in
Fe[N(SiMe3)213 suggests stronger Fe-N bonding in this complex compared with 1 but an
examination of the available crystallographic data would appear to indicate the opposite bonding
picture (Fe-N in Fe[N(SiMe 3 )2 ] 3 = 1.92 A, 15 Fe-N in 1 = 1.84 A). However, the large errors
associated with the bond lengths in 1 probably render such a comparison meaningless.
The reaction that produces 1 is relatively complex and is sensitive to a number of factors
including temperature and solvent. For example, if the reaction is carried out at room temperature
in THF the main product isolated is [N3 N]Mo(N 2 ) suggesting that oxidation of
{[N3 N]Mo(N 2 ) 2 Mg(THF)2 occurs exclusively (equation 2). If THF is employed as the solvent
and the reaction is conducted at low temperature, the major species produced is "{ [N3 N]MoN=N}2 Fe(THF)2" (see below) according to 1H NMR spectroscopy.
References begin on page 102
Chapter2
{ [N3N]Mo-N=N} 2 Mg(THF) 2 + FeC
2
THF
It
2 [N3 N]Mo(N 2 ) + Fe°
(2)
+ MgC12
Toluene and pentane solutions of 1 are an intense purple color whereas THF solutions are
orange-brown in color. As noted previously, the UV-visible spectrum of 1 in pentane has an
intense absorption at 516 nm (E= 22,800 M- 1 cm- 1) that shifts to 476 nm upon addition of THF.
1H
NMR spectroscopy was used to determine the nature of this color change. The lower half of
Figure 2.4 shows a portion of the 1H NMR spectrum of 1 in C6 D6 . The resonance at 9.25 ppm is
assigned to the TMS groups of the TREN ligand and the resonance at -9.71 ppm is attributed to
one set of the methylene protons of the ligand backbone. The relatively sharp resonance at -4.5
ppm is due to the presence of a small amount of [N3N]Mo(N 2 ) in the sample. Upon addition of 10
equivalents of THF-d8 to the sample, a color change from purple to orange-brown is discernible
and the upper spectrum in Figure 2.4 is obtained. It is seen that the resonance at 9.25 ppm has
decreased significantly in intensity and a new resonance at 6.57 ppm has grown in. Furthermore,
the resonance assigned to [N3 N]Mo(N 2 ) has increased dramatically in intensity. This result
suggests that coordination of THF to the iron center effects Fe-N bond homolysis thereby reducing
Fe(lI) to Fe(II) and induces the extrusion of an equivalent of [N3N]Mo(N 2 ) (as evidenced in the
1H
NMR spectrum by the increase in the intensity of the resonance at -4.5 ppm), yielding a
tetrahedral Fe(II) complex tentatively formulated as {[N 3 N]Mo(N 2 )} 2 Fe(THF) 2 (equation 3).
This reaction is reversible and it has been shown by 1H NMR spectroscopy
that
{ [N3 N]Mo(N 2 ) 12 Fe(THF) 2 reacts with [N3 N]Mo(N 2 ) to give free THF and 1. Similar redox
behavior has been found to occur in vanadium/molybdenum dinitrogen complexes (see below).
+ THF
{ [N 3N]Mo-N=N) 3Fe -
References begin on page 102
-THF
{[N3N]Mo-N=N2Fe(THF) 2 + [N3N]Mo(N 2 )
(3)
Chapter2
{[N3N]Mo(N 2) 2Fe(THF) 2
+ THF-d8
[N 3N]Mo(N 2)
{[N3N]Mo(N 2) 3Fe
I
&
I
°
,
I
10
0 )
i
I
D
-10
Figure 2.4. 1H NMR spectrum of { [N3N]Mo-N=N} 3Fe (lower spectrum) and 1H NMR spectrum
of {[N3N]Mo-N=N} 3Fe after addition of 10 equivalents of THF-d8 (upper spectrum).
References begin on page 102
Chapter2
Two plausible mechanisms for the formation of 1 are shown in Scheme 2.2. In the first,
nucleophilic substitution and redox reactions occur at similar rates to generate
{[N3 N]Mo(N 2 ) 2 Fe(THF)2 and [N3 N]Mo(N 2 ) in solution. {[N3N]Mo(N 2 )1 2 Fe(THF) 2 then
reacts with [N3 N]Mo(N 2 ) to yield 1. Alternatively, if the rate of nucleophilic substitution'is faster
than the rate by which {[N3 N]Mo(N 2 )1 2 Mg(THF) 2 is oxidized then "{ ([N3 N]Mo(N 2 )) 3 Fe {MgCl}+" might be generated in situ and subsequently oxidized by FeC12 to give 1. Interestingly,
the related Fe(III) complex Fe(NRAr)3 is synthesized by oxidation of the "ate" complex
(ArRN)Fe(gp-NRAr)2Li(OEt2).
14
The observations that reaction of {[N3N]Mo(N 2 ) }2Mg(THF)2
and FeC12 at room temperature yields [N 3 N]Mo(N 2 ) and that the species proposed to be
{[N 3 N]Mo(N 2 )}2 Fe(THF) 2 reacts with [N3 N]Mo(N 2 ) to give free THF and 1, suggest that
formation of 1 occurs by the first mechanism. Therefore, it appears that isolation of 1 is
contingent on the facility of {[N3 N]Mo(N 2 ) }- to act as both a nucleophile and a reductant.
Attempts to find a more direct route to 1 have been unsuccessful. Addition of FeC13 to
THF solutions of {[N3 N]Mo(N2)} 2 Mg(THF) 2 at -20 *C results in a vigorous reaction yielding
complex product mixtures. Among the products identifiable by 1H NMR spectroscopy are
[N3 N]MoCl, [N3 N]Mo(N 2 ) and {[N 3 N]Mo(N 2 )12 Fe(THF) 2 , along with 1 and unreacted
{[N3 N]Mo(N 2 ) }2 Mg(THF) 2.
Efforts to isolate {[N3 N]Mo(N 2 ) 2 Fe(THF) 2 were unsuccessful presumably due to the
lability of the THF ligands. Isolation of an Fe(II) complex was effected by replacement of the
THF ligands with a chelating diphosphine, dimethylphosphinoethane (DMPE). Addition of
DMPE to a toluene solution of 1 produces {[N3 N]Mo-N=N} 2 Fe(DMPE) (2) that can be isolated
as black blocks from diethyl ether in moderate yield (43% based on the number of equivalents of
{[N3N]Mo(N 2 ))}2Mg(THF) 2 used to generate 1) (equation 4).
{[N 3N]Mo-N=N} 3 Fe
References begin on page 102
DMPE
toluene
-
{[N 3N]M -N=N} 2 Fe(DMPE) + [N3N]Mo(N 2 )
2
(4)
Chapter2
Scheme 2.2. Possible mechanisms for the formation of { [N3 N]Mo(N 2 )13 Fe (1).
{[N3 N]Mo(N 2 )}2 Mg(TF)2 + FeC12
0.5 {[N3N]Mo(N 2 ) 2 Mg(THF) 2 + 0.5 FeC12
- 0.5 Fe"
- MgC12
{[N3 N]Mo(N 2) }2 Fe(THF) 2
- 0.5 MgC12
+
[N3N]Mo(N 2)
{[N3N]Mo(N 2 ) 3 Fe
1
+ 0.5 FeC12
-0.5 FeO, - 0.5 MgC12
"{([N3N]Mo(N 2))3Fe }- {MgCI)+ "
- 0.5 MgC12
1.5 { [N 3N]Mo(N 2 ) 2 Mg(THF) 2 + FeC12
References begin on page 102
Chapter2
A preliminary X-ray study of 2 established the connectivity, showing it to be a tetrahedral
iron complex but a disorder in the trimethylsilyl groups of the [N3 N] 3 - ligand prevented
satisfactory refinement. 22 The 1H NMR spectrum of 2 in C6 D6 consists of five broad resonances
between +40 ppm and -118 ppm. The [N3 N]Mo portion of 2 appears C3-symmetric and two
broad resonances at +37.00 ppm and -117.46 ppm are ascribed to the methyl and methylene
protons of the DMPE ligand. Due to the proximity of the phosphorus nuclei to the paramagnetic
iron center a resonance could not be located in the 3 1P NMR spectrum of 2. The IR spectrum of 2
has a strong, sharp band at 1706 cm- 1 that is assigned to VNN and is consistent with the
formulation of 2 as a diazenido species. The UV-visible spectrum of 2 has two intense
absorptions at 360 nm (e = 23,306 M- 1 cm- 1) and 508 nm (E = 13,997 M- 1 cm- 1) that are
unaffected by the addition of THF (in contrast to the behavior of 1). 2 apparently decomposes
rapidly in the solid state when exposed to high vacuum as evidenced by a color change from purple
to dark brown. We speculate that loss of DMPE is the first step in this decomposition although no
products of the reaction have been identified.
SQUID magnetic susceptibility studies have been carried out on solid 2 and the data can be
fit to the Curie-Weiss law (x = pt2/8(T-0)) over the temperature range 50-300 K to yield t=
5.08(3) 91B, 0 = 2.4(6) K, consistent with a system containing four unpaired electrons. A
M6ssbauer study of 2 was undertaken to unequivocally establish the oxidation state and spin state
of iron. The M6ssbauer spectrum of 2, taken at 77 K, is shown in Figure 2.5. The appearance of
a symmetric quadrupole doublet and the magnitude of the parameters associated with it (quadrupole
splitting = 1.15 mm/sec and isomer shift = 0.67 mm/sec) are fully consistent with the formulation
of 2 as a high-spin Fe(II) complex. 19
In an attempt to improve the yield of 2,
[DMPE]FeC12 was reacted with
{[N3 N]Mo(N 2 ) }2Mg(THF)2 in THF. Over the course of 12 h the color of the reaction mixture
turned deep green and then purple as a mixture of 2 and [N3 N]Mo(N 2 ) was formed. 2 could not
be isolated in good yield by this method and this reaction illustrates the delicate balance required to
favor metathesis over redox chemistry in these systems.
References begin on page 102
540
520 '
I-.
500
VELOCITY (mm/sec) RELATIVE TO NATURAL IRON FOIL
Figure 2.5. Mdssbauer spectrum of { [N3N]Mo-N=N} 2Fe(DMPE) (2) at 77 K.
Chapter2
Vanadium/Molybdenum Dinitrogen Complexes
Spurred on by the successful isolation of iron/molybdenum dinitrogen complexes, a study
of related vanadium complexes was initiated. Initial efforts focused on the reaction of VC13 (THF)3
with {[N3 N]Mo(N 2 ) 2 Mg(THF) 2 with a view to preparing a trigonally symmetric vanadium
complex analogous to 1. However, as discussed below, the chemistry is complicated by redox
reactions but by drawing on the experience gained in the iron system it has been possible to isolate
examples of V(mI) and V(IV) heterometallic dinitrogen complexes.
VC14 (DME) reacts with 1.5 equivalents of {[N3 N]Mo(N 2 )}2 Mg(THF)2 in THF to yield
deep purple solutions.
1H
NMR spectra of the crude reaction mixture reveal the presence of two
paramagnetic and one diamagnetic species as well as traces of [N3 N]Mo(N 2 ), [N3 N]MoH and
[bitN 3 N]Mo (see Chapter 3). Paramagnetic {[N3N]Mo(N 2 )13VC1 (3) can be separated from the
reaction mixture by crystallization from THF/pentane as black plates and is isolated in 42% yield.
The actual yield of 3 is higher according to 1 H NMR spectra of the mother liquor but efforts to
increase the isolated yield have been unsuccessful. The connectivity of 3 has been established by a
single crystal diffraction study of low resolution (1.6 A) which clearly shows three [N3 N]Mo(N2)
ligands and one chloride ligand bound to vanadium. 22 The 1H NMR spectrum of 3 consists of
three relatively sharp resonances at 1.36 (Av1/ 2 = 14 Hz), 0.94 (Av 1/ 2 = 6 Hz) and -0.55 ppm
(Avl/2 = 14 Hz) with the resonance at 0.94 ppm being assigned to the TMS groups of the ligand.
Complex 3 appears stable in solution and C6 D6 solutions of 3 remain unchanged when stored
under dinitrogen for a period of days (according to 1H NMR spectroscopy). The IR spectrum of 3
in Nujol exhibits a broad N-N stretch at 1579 cm- 1 and the UV-visible spectrum of 3 in pentane
has an intense absorption at 540 nm (s = 29,787 M- 1 cm-1). SQUID magnetic susceptibility
measurements on solid 3 are in accord with its formulation as a dl V(IV) complex and the data is
plotted in Figure 2.6. Fitting the data to the Curie law yields t = 1.50(1) tB (R = 0.9998).
The second paramagnetic species that is present in the reaction mixture is characterized by
broad, unassigned resonances at 6.00, 1.28 and -5.56 ppm. Although this complex has not been
isolated, it is formulated as the V(III) complex {[N3N]Mo(N 2 )}2 VCI(THF) (4) on the basis of
References begin on page 102
Chapter2
investigations of reactions between VC13 (THF) 3 and {[N3 N]Mo(N 2 ) 2 Mg(THF) 2 (see below).
The diamagnetic species appears to be the diazenido complex [N3 N]Mo-N=N-TMS with a pair of
triplets being observed at 3.38 and 2.10 ppm in addition to a singlet at 0.50 ppm.
Figure 2.6. Plot of Xm (corrected for diamagnetism using Pascal's constants) versus T for
{ [N3 N]Mo-N=N} 3 VCI, (4).
0.06
0.05
0.04
X m 0.03
0.02
0.01
0
50
100
150
200
250
300
350
T (K)
Reaction of VC13 (THF) 3 with one equivalent of {[N3 N]Mo(N 2 ) }2 Mg(THF) 2 also yields
deep purple solutions from which 3 can be isolated in 28% yield. The formation of 3 was
unanticipated and suggests that a fraction of VC13 (THF)3 is being reduced during the course of the
reaction although no reduced vanadium species has been isolated from the reaction mixture.
Furthermore, 1H NMR spectra of the crude reaction mixture are complex revealing the presence of
3 and 4 along with traces of [N3 N]MoH, [bitN 3 N]Mo and [N3 N]Mo(N 2 ). Reaction of
VC13 (THF) 3 with 1.5 equivalents of {[N 3 N]Mo(N 2 )}2Mg(THF)
2
does not yield the trigonal
planar complex { [N3 N]Mo(N 2 )13 V. Red-purple solutions are obtained (as opposed to the deep
References begin on page 102
Chapter2
purple color observed in previous reactions) and 1H NMR spectra again reveal the presence of 3
and 4 as well as trace amounts of [N3 N]MoH, [bitN3N]Mo and [N3N]Mo(N2). Resonances at
-0.55 ppm (3) and -5.56 ppm (4) integrate to a ratio of 1:2 suggesting that 4 is formed in higher
yield than in the reaction of VC14 (DME) and 1.5 equivalents of { [N3 N]Mo(N 2 )}2Mg(THF)2
where the resonances integrate to a ratio of approximately 2:1. Upon standing under dinitrogen for
a period of hours, C6 D 6 solutions of the mixture of 3 and 4 take on a deep purple color as 3
becomes the major species present in solution. 1H NMR spectra of the purple solutions reveal that
the resonances attributable to 4, most noticeably those at 1.28 and -5.56 ppm, have diminished in
intensity with a concomitant increase in intensity of the resonances for 3. These results clearly
indicate that 4 is unstable in solution, undergoing a disproportionation reaction that yields 3 as one
of the products. Unfortunately, no other products of this reaction have been identified.
By analogy to the reaction of {[N 3 N]Mo(N 2 )}2 Fe(THF) 2 with [N3 N]Mo(N 2 ) that
produces 1, it was reasoned that 4 should react with [N3 N]Mo(N 2 ) to yield 3 (equation 5). The
1H
NMR spectrum of the mixture of 3 and 4 is shown in Figure 2.7 (lower spectrum) along with
the 1 H NMR spectrum of a sample to which [N3 N]Mo(N 2 ) has been added. Upon addition of
[N3N]Mo(N2) to the mixture of 3 and 4 an immediate color change to deep purple is observed and
resonances attributable to 4 are no longer present in the 1H NMR spectrum while those of 3 have
increased significantly in intensity (upper spectrum, Figure 2.7). These results are consistent with
coordination of [N3 N]Mo(N2) and oxidation of the vanadium center. The reverse reaction does
occur but the equilibrium lies far to the right. Upon addition of THF-d 8 (- 400 eqs) to C6 D 6
solutions of 3 resonances attributable to trace amounts of [N3N]Mo(N 2 ) and 4 are observed in the
1H
NMR spectrum but after 24 h the main species present in solution is 3.
{ [N3 N]Mo(N 2) }2 VC(THF) + [N3N]Mo(N 2 )
C6 D 6
-THF
([N
3N]Mo(N 2)}3 VCl
(5)
3
4
In an attempt to synthesize { [N3 N]Mo(N 2 )} 4 V, VC14 (DME) was reacted with two
equivalents of { [N3 N]Mo(N 2 )} 2 Mg(THF) 2 in THF at -20 oC.
References begin on page 102
1H
NMR spectra of the crude
S{ [N3N]Mo(N2)
3VCI (3)
-t
5
0
[N3N]Mo(N 2)
O
* = ( [N3N]Mo(N 2 )) 2VCI(THF) (4)
lii
1 1l171 11111
8
l1
6
I
1 11l
4
ll 11
r7 1 1 111
2
11
-0
1ll
11
-2
Illilrl till
-4
Ill
ll
-6
11 1
11
1 111111 1111
-8
ppm
Figure 2.7. 'H NMR spectrum of a mixture of 3 and 4 in C6D6 (lower spectrum) and 'H NMR spectrum of a sample
to which [N3N]Mo(N 2) was added (upper spectrum).
t,,*1
Chapter2
reaction mixture reveal that 3 and 4 are formed along with traces of [N3 N]MoH and [bitN 3 N]Mo.
It is believed that {[N3 N]Mo(N2) }4 V is not accessible for steric reasons and the synthesis of this
complex was not pursued further.
Although the instability of 4 in solution has precluded its isolation on a preparative scale,
crystals of 4 suitable for an X-ray diffraction study were grown from THF/Et20 solutions of the
reaction mixture at -20 'C.
1.25 molecules of diethyl ether were found in the unit cell.
Crystallographic data and collection and refinement parameters are given in Table 2.1. The
molecular structure of 4 along with the atom-labeling scheme is shown in Figure 2.8 while selected
bond lengths and bond angles are listed in Table 2.4. 4 is comprised of two {[N3 N]Mo(N 2 )
-
units bound to pseudo-tetrahedral vanadium, the coordination sphere being completed by one
molecule of THF and one chloride ligand. The N-V-N bond angle opens to 119.80 in order to
accommodate the sterically bulky {[N3 N]Mo(N 2 ) }- ligands. The Mo-N-N linkages are essentially
linear and the N-N bond lengths at 1.217(7) and 1.221(7) A are indicative of some reduction of the
dinitrogen ligands in 4 compared with free dinitrogen (1.098 A) and are consistent with
formulation of 4 as a diazenido complex with Mo and V in formal oxidation states of 4+ and 3+,
respectively. The Nax-Mo-Neq-Si dihedral angles are all close to 180', suggesting that there is little
steric pressure in the pocket defined by the [N3 N] 3- ligand. The first structurally characterized
vanadium dinitrogen complex 23 was reported in 1989 but examples of V(I) dinitrogen complexes
remain comparatively rare.
The homobimetallic complexes [(Np)3V]2(Ig-N
2 ),
24
[CH 3 C {(CH2)N(ipr) }3V]2(-N2)25 and [(iPr2N)3V]2(-N2) 26 have been crystallographically
characterized but 4 is the first example of a heterometallic vanadium dinitrogen complex. The N-N
bonds in 4 are slightly shorter than the corresponding bonds in the homobimetallic complexes (1.26 A). Interestingly, the V-N bond lengths of the homobimetallic complexes (- 1.72 A) are
significantly shorter than those in 4 (1.86 A).
These bonding parameters suggest that the
homobimetallic complexes are perhaps better formulated as V(IV) or V(V) complexes with the
dinitrogen ligand reduced to the diazenido or hydrazido stage.
References begin on page 102
i~0~
~
i_ --
i~L111
I -- i~
-_
sr~ -q
__11
_~
I
Chapter2
Figure 2.8. A view of the structure of {[N 3 N]Mo-N=N} 2 VCl(THF) (4).
Table 2.4. Selected bond lengths and bond angles for {[N3N]Mo-N=N} 2 VCI(THF) (4).
Bond Lengths (A)
N(1)-N(2)
1.217(7)
N(3)-N(4)
1.221(7)
Mo(1)-N(1)
1.836(6)
Mo(2)-N(3)
1.827(6)
V-N(2)
1.864(4)
V-N(4)
1.860(6)
Mo(1)-N(14) 2.244(6)
V-Cl
V-o
2.288(2)
2.061(5)
Bond Angles (deg)
Mo(1)-N(1)-N(2)
178.2(5)
Mo(2)-N(3)-N(4)
V-N(2)-N(1)
169.2(5)
V-N(4)-N(3)
C1-V-N(2)
114.9(2)
O-V-Cl
References begin on page 102
172.1(5)
96.5(2)
178.3(5)
N(2)-V-N(4)
119.8(3)
O-V-N(4)
104.2(2)
la-
Chapter2
Zirconium/Molybdenum Dinitrogen Complexes
From our excursions into iron and vanadium chemistry, coupled with the observation that
reactions of { [N3 N]Mo(N 2 ) 2 Mg(THF) 2 with halides of transition metals such as palladium,
nickel and zinc proceed via oxidative pathways yielding [N3 N]Mo(N 2 ) (see Chapter 1), it is clear
that the tendency for redox chemistry to prevail increases as we move to the right of the transition
metal series. Drawing on these results it seemed reasonable that redox chemistry might be avoided
by employing halides of earlier transition metals such as zirconium. This approach has been
successful and several zirconium/molybdenum dinitrogen complexes have been isolated.
ZrC14 (THF)2 proved to be a versatile reagent for the synthesis of zirconium/molybdenum
heterometallic dinitrogen complexes and { [N3 N]Mo(N 2 ) }2Mg(THF) 2 reacts cleanly with two
equivalents of ZrCl 4 (THF)2 in THF to give {[N3 N]Mo-N=N }ZrC13 (THF)2 (5) as salmon-colored
needles in 77% yield (equation 6). The 1H NMR spectrum of diamagnetic 5, taken in THF-ds,
consists of a single TMS resonance and a pair of triplets for the methylene protons on the ligand
backbone characteristic of compounds in which the [N3N]Mo portion of the molecule is C3 symmetric. Resonances attributed to coordinated THF are also observed in the spectrum and
elemental analyses are consistent with there being two molecules of THF present per zirconium
center. The IR spectrum of 5 in Nujol has a broad N-N stretch at 1515 cm- 1 which is within the
range reported for related Group 4 heterobimetallic bridging dinitrogen complexes (1468 - 1545
cm-1). 5
{[N 3N]Mo-N=N) 2 Mg(THF) 2 + 2 ZrCl4 (THF)2
THF _ 2 {[N 3N]Mo-N=N }ZrCl 3 (THF)2
5
+ MgC12
(6)
By varying the stoichiometry of the reaction depicted in equation 6, two other
zirconium/molybdenum dinitrogen complexes can be isolated. Reaction of one equivalent of
References begin on page 102
Chapter2
{[N3 N]Mo(N2) }2 Mg(THF)2 with one equivalent of ZrCl 4 (THF) 2 yields the diamagnetic complex
{[N 3 N]Mo(N2)1 2 ZrC12 (6) as red cubes in moderate yield (54%, equation 7). The 1H NMR
spectrum of 6 in C6 D6 reveals the presence of one equivalent of THF per zirconium center, an
observation that is corroborated by elemental analysis data. The IR spectrum of 6 in Nujol is
characterized by a strong N-N stretch at 1556 cm- 1. 6 is unstable in solution undergoing a ligand
redistribution reaction that produces 5 and {[N3 N]Mo(N 2 )}3 ZrCI (7, see below) and after 48 h 1H
NMR spectra of C 6 D6 solutions indicate that 5, 6 and 7 are present in an approximate ratio of
1:3:1.
{[N 3N]Mo-N=N} 2Mg(THF) 2 + ZrC14(THF)
2
THF
{[N 3N]Mo-N=N} 2ZrCl 2
6
(7)
+ MgC12
Single crystals of 6 were grown from saturated diethyl ether solutions at -20 OC and
examined in an X-ray study; a half a molecule of diethyl ether was found in the unit cell.
Crystallographic data and collection and refinement parameters are given in Table 2.5. The
molecular structure of 6 along with the atom-labeling scheme is shown in Figure 2.9 while selected
bond lengths and bond angles are listed in Table 2.6. The data confirm that two [N3 N]Mo(N 2 )
ligands are coordinated to pseudo-tetrahedral zirconium with two chloride ligands completing the
coordination sphere. The Mo-N-N and Zr-N-N linkages are essentially linear and the large size of
Zr(IV) and its ability to accommodate the sterically bulky [N3N]Mo(N 2 ) ligands is reflected in the
N(16)-Zr-N(26) bond angle of 114.6(2)0. The Mo-Na bond lengths at 1.797(6) and 1.796(6) A
are the shortest of all the crystallographically-characterized heterometallic complexes reported in
this chapter, suggesting extensive dn-pn multiple bonding between these atoms. We can assign
oxidation states of 4+ to the molybdenum and zirconium centers and so 6 is formally a diazenido
complex with dinitrogen functioning as a (N2 )2- ligand. The N-N bond lengths in 6 straddle the
ranges reported for bimetallic diazenido (1.20-1.25
References begin on page 102
A) and
hydrazido (1.25-1.34 A) complexes 2
Chapter2
and are comparable to the N-N bond length of 1.24(2) A in the related heterobimetallic dinitrogen
complex WI(PMe2Ph)3(py)(g-N2)ZrCp2C1.
5
In 6 the TMS groups are all oriented upright with
the Nax-Mo-Neq-Si dihedral angles close to 1800, consistent with minimal steric pressure within the
pocket. Finally, the Mo-Namido bond lengths are not statistically different and are similar to MoNamido bond lengths in many other triamidoamine complexes. 27
Figure 2.9. A view of the structure of {[N 3 N]Mo-N=N} 2 ZrC12 (6).
References begin on page 102
Chapter2
Table 2.5. Crystallographic data, collection parameters and refinement parameters for
{[N3 N]Mo-N=N} 2 ZrCl2 (6).
6
Empirical Formula
C32 H88 C12Mo 2 N 1200.50 Si 6 Zr
Formula Weight
1171.68
Diffractometer
SMART/CCD
Crystal Dimensions(mm)
na
Crystal System
Monoclinic
Space Group
P21/c
a(A)
16.4150(3)
b (A)
18.5686(4)
c (A)
19.8964(4)
a (0)
90
100.2590(10)
A(
(0)
90
V (A3), Z
5967.5(2), 4
Dcalec (Mg/m3)
1.304
Absorption coefficient (mm-l)
0.829
Fooo
2420
Temperature (K)
183(2)
8 range for data collection (0)
1.26 to 23.29
Reflections collected
23769
Unique Reflections
8552
R
0.0559
Rw
0.0986
GoF
1.056
References begin on page 102
Chapter2
Table 2.6. Selected bond lengths and bond angles for {[N3 N]Mo(N 2 )}2 ZrC12 (6).
Bond Lengths
(A)
Mo(1)-N(15) 1.797(6)
Mo(2)-N(25)
1.796(6)
N(15)-N(16)
1.249(8)
N(25)-N(26)
1.245(8)
Zr-N(16)
1.978(6)
Zr-N(26)
1.974(6)
Zr-Cl(1)
2.394(2)
Zr-Cl(2)
2.408(2)
Mo(1)-N(14) 2.236(6)
Mo(2)-N(24) 2.251(6)
Bond Angles (degrees)
Mo(1)-N(15)-N(16)
176.9(5)
Mo(2)-N(25)-N(26)
177.9(5)
Zr-N(16)-N(15)
175.9(6)
Zr-N(26)-N(25)
170.6(5)
N(16)-Zr-N(26)
114.6(2)
Cl(1)--Zr-Cl(2)
107.14(9)
Dihedral Angles (degrees)a
N(14)-Mo(1)-N(13)-Si(13)
174.4 N(24)-Mo(2)-N(21)-Si(21)
-173.6
aObtained from a Chem 3D drawing
To further probe the ability of Zr(IV) to accommodate the sterically bulky [N3 N]Mo(N 2 )
ligand, ZrC14 (THF)2 was reacted with 1.5 equivalents of {[N3 N]Mo(N 2 ) }2 Mg(THF) 2 according
to equation 8. {[N3N]Mo(N 2 )13ZrC1 (7) can be isolated as deep red needles from diethyl ether in
68% yield. Similar to 5 and 6 above, 1 H and
13 C
NMR spectra of 7 are characteristic of a
complex in which the [N3N]Mo portion of the molecule is C3-symmetric. The IR spectrum of 7
taken in Nujol has a strong broad stretch at 1576 cm - 1 . 7 appears to be thermally stable and C6 D6
solutions of 7 show no signs of decomposition when stored at room temperature under dinitrogen
for 72 h (according to 1H NMR spectroscopy).
References begin on page 102
Chapter2
1.5 { [N3N]Mo-N=N} 2Mg(THF) 2 + ZrCl4(THF) 2
Et20/tol
0-
{[N 3N]Mo-N=N} 3 ZrCl
7
(8)
+ 1.5 MgC12
Efforts to prepare {[N3 N]Mo(N 2 )14Zr were unsuccessful; reaction of ZrC14(THF) 2 with
two equivalents of {[N 3 N]Mo(N 2 )} 2 Mg(THF)2 yielded 7 as the sole identifiable product
suggesting that {[N3N]Mo-N=N} 4 Zr is not accessible on steric grounds.
CONCLUSIONS
The concept of {[N3 N]Mo(N 2 ) }- as a ligand has been explored and implemented in the
synthesis of heterometallic dinitrogen complexes. The { [N3 N]Mo(N 2 )}- ligand is unique not only
in that it is derived from dinitrogen but also in its ability to exist in both anionic and neutral forms.
The non-innocent nature of the ligand coupled with the presence of a redox active metal center
facilitates the interconversion of Fe(II)/Fe(LII) and V(III)/V(IV) dinitrogen complexes as evidenced
by 1 H NMR spectroscopic studies. It is clear that upon coordination of [N3 N]Mo(N 2 ) to the
Fe(II) or V(III) center, an electron is transferred from the metal to the ligand, and as a result
complexes containing the neutral ligand have not been isolated. Reduction of the metal center with
the concomitant oxidation of {[N3N]Mo(N 2 ) }- has thwarted efforts to synthesize heterometallic
complexes containing later transition metals although such reactions do allow isolation of the
neutral terminal dinitrogen complex [N3 N]Mo(N 2 ) (see Chapter 1). Complexes containing four
{[N 3 N]Mo(N 2 ) }- ligands have not been isolated and it is proposed that the steric bulk of the ligand
prevents the formation of such complexes.
References begin on page 102
Chapter2
EXPERIMENTAL PROCEDURES
General Details. All experiments were performed under a nitrogen atmosphere in a
Vacuum Atmospheres drybox or by standard Schlenk techniques unless otherwise specified.
Pentane was washed with sulfuric acid/nitric acid (95/5 v/v), sodium bicarbonate, and water,
stored over calcium chloride, and distilled from sodium benzophenone ketyl under nitrogen.
Toluene was distilled from sodium, and CH 2 C12 was distilled from CaH 2 . Anhydrous diethyl
ether and THF were sparged with nitrogen and passed through alumina columns. 2 8 All solvents
were stored in the dry box over activated 4 A molecular sieves.
NMR data were obtained at 300 or 500 MHz ( 1H), 75.4 MHz (13 C) and 121.8 MHz ( 3 1p)
and are listed in parts per million downfield from tetramethylsilane for proton and carbon and in
parts per million downfield from 85% H3 PO4 for phosphorus. Coupling constants are listed in
Hertz. Spectra were obtained at 25 OC unless otherwise noted. Benzene-d6 and toluene-d8 were
pre-dried on CaH2 , vacuum transferred onto sodium and benzophenone, stirred under vacuum for
two days and then vacuum transferred into small storage flasks and stored over molecular sieves.
[N 3 N]MoC1, 9 [DMPE]FeCl 2 ,2 9 VCl 3 (THF) 3 ,3 0 VCL 4 (DME) 3 1 and ZrC14 (THF) 23 0 were
prepared as described in the literature. PdC12 (PPh 3 )2 , NiCl 2 (PPh 3 )2 , FeC12 and FeC13 were
purchased from commercial vendors and used as received.
UV/visible spectra were recorded on a HP 8452 Diode Array spectrophotometer using a
Hellma 221-QS quartz cell (path length = 10 mm) sealed to a gas adapter fitted with a Teflon
stopcock. IR spectra were recorded on a Perkin-Elmer 1600 FT-IR spectrometer. Elemental
analyses (C, H, N) were performed in our laboratory using a Perkin-Elmer 2400 CHN analyzer or
by Microlytics Analytical Laboratories of Deerfield MA. X-ray data were collected on Siemens
SMART/CCD diffractometer and general experimental details are described in the literature. 32
SQUID Magnetic Susceptibility Measurements. Measurements were carried out
on a Quantum Design SQUID magnetometer. Data were obtained at a field strength of 5000
Gauss. Straws and gel caps (Gelatin Capsule No. 4 Clear) were purchased from Quantum Design.
The sample was prepared in the drybox by the following method. A gel cap and a square of
References begin on page 102
Chapter2
parafilm were weighed. The sample was placed in the gel cap and the parafilm inserted above it.
The gel cap was closed and the mass of the sample was ascertained by weighing the loaded gel
cap. The gel cap was placed in a straw which was then mounted on the sample rod and placed in
the magnetometer. Two runs were performed on the sample - one from 5 to 300 K and a second
from 300 to 5 K. Measurements were made at the following increments: 5-10 K (every 1 K), 1020 K (every 2 K), 20-50 K (every 3 K), 50-100 K (every 5 K), 100-200 K (every 10 K), 200-300
K (every 20 K).
{[N 3 N]MoN 2 }3 Fe (1).
{[N 3 N]Mo(N2)} 2 Mg(THF) 2 (316 mg, 0.28 mmol) was
dissolved in 10 mL Et 2 0/ 2 mL THF/ 1 mL toluene and the solution was cooled to -20 OC. FeC12
(35 mg, 0.28 mmol) was slurried in 1 mL Et 2 0, cooled to -20 OC and then was added all at once to
the stirred solution of {[N3N]Mo(N 2 ) }2 Mg(THF) 2 . Over the course of 15 min FeC12 was taken
into solution as the color of the solution darkened to a burnt orange color. After 90 min the solvent
was removed to give a black purple residue. This residue was extracted with 30 mL of pentane
and the purple solution was filtered through Celite. Upon reducing the pentane solution purple
crystals began to form. The solution was cooled to -20 OC and the product obtained as a blackpurple crystalline solid; yield 105 mg (38%)
1H
NMR (C6 D6 ) 8 9.25 (Avl/2 = 705 Hz, TMS),
-9.71 (AvI/2 = 288 Hz, NCH 2 CH 2 N), -64.0 (Av1/ 2 = 241 Hz, NCH2CH 2 N). IR(Nujol, cm - 1)
1703 (N=N). UV-visible(Pentane) X= 516 nm, e = 22,818 M- 1 cm- 1 . t = 6.03 J-B.
{[N 3 N]MoN 2 12 Fe(DMPE).ether (2). {[N 3 N]Mo(N 2 )} 2 Mg(THF) 2 (454 mg, 0.40
mmol) was dissolved in 10 mL of THF and the solution was cooled to -20 OC. FeC12 (51 mg,
0.40 mmol) was added all at once to the stirred solution. After 1 min the solution began to darken
in color and took on a dark burnt-orange color. After 45 min the solvent was removed in vacuo
and the residue extracted with 40 mL of pentane. The purple pentane solution was filtered through
Celite and the pentane removed to give a black/purple solid. This solid was dissolved in 10 mL of
toluene and DMPE (60 mg, 0.40 mmol) in 3 mL of toluene was added dropwise to the stirred
solution. After stirring for 20 min the solvent was removed in vacuo. The residue was extracted
with 15 mL of diethyl ether, filtered and the volume of the filtrate was reduced to 7 mL. Upon
References begin on page 102
Chapter2
cooling this solution to -20 °C the product was obtained as black blocks; yield 200 mg (43%). 1H
NMR (C6 D6 ) 8 37.0 (P(CH3 )2 or PCH2), 6.62 (Av 1/2 = 690 Hz, TMS), -4.67 (Av 1/2 = 337 Hz,
NCH 2 CH 2 N), -47.43 (Avl/ 2 = 591 Hz, NCH 2 CH 2 N), -117.46 (P(CH3 )2 or PCH 2 ). IR(Nujol,
cm- 1 ) 1706 (N=N). UV-visible(Pentane) , = 360 nm, e = 23,306 M- 1 cm- 1 , k = 508 nm, e =
13,997 M- 1 cm - 1. i = 5.08 gB. Anal. Calcd. for C4 0 H 104N 12 Si 6 Mo 2 FeP 2 0: C, 38.51; H, 8.40;
N, 13.47. Found: C, 38.55; H, 8.43; N, 13.53.
{[N 3 N]MoN 2 }3VCI (3). Method 1. {[N 3 N]Mo(N 2 )}2 Mg(THF) 2 (300 mg, 0.264
mmol) was dissolved in 10 mL THF and cooled to -20 'C. VC14 (DME) (50 mg, 0.177 mmol) was
dissolved in 3 mL THF, cooled to -20 °C and then added to the stirred solution of
{[N3N]Mo(N2)12Mg(THF)2. The solution was stirred for 15 h to give a purple solution. The
solvent was removed and the residue extracted with 15 mL of toluene. Following filtration
through Celite, the toluene was removed in vacuo and the resulting residue was recrystallized from
THF/pentane to give the product as black plates; yield 112 mg (42%).
Method 2. { [N3 N]Mo(N 2 ) 2 Mg(THF)2 (290 mg, 0.255 mmol) was dissolved in 7 mL
THF and cooled to -20 °C. VCl 3 (THF) 3 (95 mg, 0.255 mmol) was dissolved in 2 mL THF,
cooled to -20 'C and then added to the stirred solution of { [N3 N]Mo(N 2 )} 2 Mg(THF) 2 . The
solution was stirred for 25 h to give a purple solution. The solvent was removed and the residue
extracted with 15 mL of toluene. Following filtration through Celite, the toluene was removed in
vacuo and the resulting residue was recrystallized from THF/pentane to give the product as black
plates; yield 108 mg (28%).
1H
NMR(C 6 D6 ) 8 1.36 (AVl/ 2 = 20 Hz, NCH 2 CH 2 N), 0.94 (Avl/2
= 6 Hz, TMS), -0.55 (Avl/2 = 14 Hz, NCH 2 CH 2 N). IR(Nujol, cm- 1) 1579 (N=N). p = 1.50
iB-
{[N 3 N]Mo(N 2 ))ZrCI 3 (THF)2 (5). {[N 3 N]Mo(N 2 ) 2Mg(THF) 2 (150 mg, 0.136
mmol) was dissolved in 7 mL of THF and cooled to -20 'C. ZrC14(THF)2 (100 mg, 0.265 mmol)
was added as a solid to the stirred solution of {[N3N]Mo(N 2 ) 2 Mg(THF) 2 . After 3 h the solvent
was removed in vacuo and the residue was extracted with 15 mL of toluene. Following filtration
through a pad of Celite, the toluene was removed under reduced pressure. Crystallization from
References begin on page 102
100
Chapter2
THF/pentane afforded the product as salmon-colored needles; yield 153 mg (77%).
1H
NMR(THF-ds) 8 3.75 (t, NCH 2 CH 2 N), 3.62 (m, THF), 2.90 (t, NCH 2 CH 2 N), 1.78 (m, THF),
0.35 (s, TMS).
13 C{(1H}
NMR(THF-ds) 8 68.4 (THF), 54.9 (NCH 2 CH 2 N), 53.4
(NCH2CH 2 N), 26.5 (THF), 4.2 (TMS).
IR(Nujol, cm - 1) 1515 (N=N). Anal. Calcd. for
C2 3 H5 5 N6 Si 3 MoZrCl302: C, 33.46; H, 6.72; N, 10.18, Cl, 12.88. Found: C, 33.90; H, 6.54;
N, 9.81, C1, 12.81.
{ [N3 N]Mo(N 2 )}2ZrCi2.THF (6). {[N 3 N]Mo(N 2 ) 2 Mg(THF)2 (300 mg, 0.264
mmol) was dissolved in 10 mL of THF and cooled to -20 OC. ZrC14(THF)2 (100 mg, 0.265
mmol) was added as a solid to the stirred solution of { [N3 N]Mo(N 2 ) }2Mg(THF) 2 . After 29 h the
solvent was removed in vacuo and the residue was extracted with toluene. Following filtration
through a pad of Celite, the toluene was removed in vacuo. Crystallization from THF/pentane
1H
afforded the product as red cubes; yield 160 mg (54%, 2 crops).
NMR(C 6 D6 ) 8 3.60 (m,
THF), 3.29 (t, NCH 2 CH 2 N), 2.04 (t, NCH 2 CH 2 N), 1.40 (m, THF), 0.65 (s, TMS).
13 C{( 1 H)
NMR(C 6 D6 ) 8 54.2 (NCH2 CH 2 N), 52.5 (NCH 2 CH 2 N), 26.1 (THF), 4.5 (NTMS). IR(Nujol,
cm - 1) 1556 (N=N). Anal. Calcd. for C34 H86 N12Si 6 Mo 2 ZrCl 2 0: C, 33.98; H, 7.21; N, 13.99.
Found: C, 33.51; H, 7.21; N, 14.00.
{[N 3 N]Mo(N 2 )}3 ZrCI (7). {[N 3 N]Mo(N2)}2Mg(THF)
2
(155 mg, 0.136 mmol) was
dissolved in 5 mL of a 2:1 ether/toluene solution and cooled to -20 oC. ZrCl4 (THF) 2 (34 mg, 0.09
mmol) was added as a solid to the stirred solution of {[N3N]Mo(N 2 )}2Mg(THF) 2 . The solution
was stirred for 17 h and then filtered through a pad of Celite to give a clear, blood-red solution.
The solvent was removed and the residue dissolved in the minimum diethyl ether. Cooling this
solution to -20 'C afforded the product as deep red needles; yield 97 mg (68%). 1H NMR(C6 D6 ) 8
3.37 (t, NCH 2 CH 2 N), 2.10 (t, NCH 2 CH 2 N), 0.69 (s, TMS).
13 C{ 1H}
NMR(C 6 D6 ) 8 55.0
(NCH 2 CH 2 N), 52.0 (NCH 2 CH 2 N), 4.7 (NTMS). IR(Nujol, cm - 1) 1576 (N=N).
References begin on page 102
101
Chapter2
REFERENCES
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(2) Hidai, M.; Mizobe, Y. Chem. Rev. 1995, 95, 1115.
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George, G. N.; Pickering, I. J. J. Am. Chem. Soc. 1996, 118, 8623 and references therein.
(4) Mercer, M. J. J. Chem. Soc., Dalton Trans. 1974, 1637.
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103
CHAPTER 3
Organometallic Chemistry of Trimethylsilyl-Substituted
Triamidoamine Complexes of Molybdenum
A portion of the material covered in this chapter has appeared in print:
Schrock, R. R., Seidel, S. W., Moisch-Zanetti, N. C., Shih, K. -Y., O'Donoghue, M. B.,
Davis, W. M., Reiff, W. M. J. Am. Chem. Soc. 1997, 119, 11876.
Chapter3
INTRODUCTION
In recent years a wide variety of transition metal complexes containing the triamidoamine
ligands [(RNCH 2 CH 2 )3N] 3 - (R = C6 F5 , Me3Si) have been synthesized in our laboratories. 1 The
salient features of such complexes are the sterically protected, three-fold-symmetric, apical
coordination site and the spatial arrangement of three orbitals of the metal center within this pocket.
The use of bulky R groups such as Me3Si is believed to impart kinetic stability on what would
otherwise be reactive species. This strategy has allowed us to isolate examples of rarely observed
complexes such as trigonal monopyramidal complexes from titanium to iron, 2 a tantalum
phosphinidene complex, 3 and tungsten and molybdenum phosphido and arsenido complexes.4 In
triamidoamine complexes the frontier orbitals of the metal that are available to bind other ligands
consist of an orbital of a symmetry (dz2) and two degenerate orbitals of 7c symmetry (dxz and dyz).
Such an orbital arrangement is ideal for the formation of M-E triple bonds as exemplified by the
synthesis of [N3 N]ME complexes (M = Mo, W; E = CR, N, P, As).1
Alternatively,
rehybridization allows the metal center to bind two ligands via a single and a double bond as is the
case in the alkylidene hydride complex [N3N]W(C 5 H8 )(H). 5 The third bonding picture in which
the metal forms single bonds to three ligands is uncommon with [N3 N]W(H) 3 5 ' 6 being a rare
example of such a complex.
One of the original drives behind our foray into the chemistry of transition metal
triamidoamine complexes was to understand how dinitrogen might be activated and reduced in a
C3-symmetric environment. In this context the exploration of the chemistry of [N3NF]Mo 7 and
[N3 N]Mo complexes has been especially rewarding and Chapter 1 details progress made toward
the derivatization of dinitrogen in [N3 N]Mo complexes. With the isolation of paramagnetic
[N3N]Mo(N 2 ), we demonstrated that a dinitrogen adduct of trigonal monopyramidal [N3 N]Mo is a
viable species. Drawing on this result, we reasoned that other [N3N]Mo(L) complexes where L =
CO, RNC, or olefin should be accessible. It should be noted that the organometallic chemistry of
molybdenum in the 3+ oxidation state is relatively little explored and that studies of the
organometallic chemistry of [N3 N]Mo complexes have focused almost exclusively on Mo(IV)
References begin on page 137
105
Chapter3
alkyl complexes and their decomposition to Mo(VI) alkylidynes, culminating with the unequivocal
demonstration that such decompositions occur via a elimination processes that are as much as six
orders of magnitude faster than 3elimination processes. 8
Two synthetic approaches to [N3N]Mo(L) complexes have been devised. In the first
approach, [N3 N]MoCl is reduced by magnesium powder in the presence of the donor ligand, L.
However, only [N3 N]Mo(C 2 H4 ) has been successfully synthesized in this manner. In the cases of
CO and tBuNC no reaction occurs and the starting material is recovered. Having observed that the
dinitrogen ligand in [N3 N]Mo(N 2 ) is labile (see Chapter 1), we reasoned that [N3 N]Mo(N 2 ) could
serve as a source of the as-yet-unobserved trigonal monopyramidal complex, [N3 N]Mo. Hence,
we embarked on a series of ligand exchange reactions leading to the high yield syntheses of
[N3 N]Mo(C 2 H4 ), [N3N]Mo(CO) and [N3 N]Mo(CNtBu). These and other reactions described in
this chapter are summarized in Scheme 3.1.
As noted previously, examples of trigonal monopyramidal triamidoamine complexes are
known for a variety of first row metals yet such complexes are unknown for second and third row
metals. Since the elusive species [N3 N]Mo is implicated in the ligand exchange reactions
(assuming a dissociative mechanism is operating), we felt it a worthy endeavor to attempt to isolate
it. However, reduction of [N3 N]MoCl in the absence of a donor ligand leads to C-H activation of
one of the trimethylsilyl groups of the ligand and [bitN 3 N]Mo is isolated. An X-ray study of
[bitN 3 N]Mo is reported, highlighting the vulnerability of the [N3 N] 3- ligand to ligand degradation
reactions other than those involving Si-N bond cleavage (see Chapter 1).
References begin on page 137
106
Chapter3
Scheme 3.1. Organometallic chemistry of [N3 N]Mo complexes.
OTMS
I
TMS
C
II
TMS
TMSMo-N
[N3N]MoC1
3
Mg
Mg, C 2H4
TMS
CH 2
SiMe2
Mg, TMSCI
TMSN I, Mo-N
NJ
TMS
0
4
TMS
TMS
TMSiy~1 -N
4
C
C
TMS NII
Mo-
s
I
/TMS
N
N,)l
N
C2H4
CO
2
[N3N]Mo(N 2)
2,6-Me 2C6H3NC
t
BuNC
tBu
Ilk
N
N
TMS
Ill
,
TMS
T MS_
C
,TMS
TN
ISNI-Mo - N
C
TMS
MNI
o - N/
4
N
TMS
,.
TMSNII
C
TMS
-
N
4%M
M
References begin on page 137
107
Chapter3
RESULTS
Synthesis of [N3 N]Mo(C 2 H 4 )
Reduction of [N3 N]MoCl in THF with an excess of magnesium powder in the presence of
5 equivalents of ethylene proceeds smoothly over a period of 12 h to give a purple solution
(equation 1). Paramagnetic [N3 N]C 2 H4 (1) can be isolated as analytically-pure, purple needles in
97% yield by recrystallization from hexamethyldisiloxane. Exposure of C6 D 6 solutions of
[N3 N]Mo(N 2 ) to an excess of ethylene (10 equivalents) results in an immediate color change from
orange to purple as 1 is formed cleanly according to 1H NMR spectroscopy. Unlike other Mo(llI)
adducts (see below) resonances for the methylene protons of the ligand backbone are not observed
in the 1H NMR spectrum of 1. Instead a single broad resonance at 3.63 ppm (Av 1/2 = 414 Hz)
assigned to the TMS groups of the ligand is observed. A resonance attributable to the ethylene
ligand is not observed, presumably due to its proximity to the paramagnetic center. Triamidoamine
complexes of ethylene are known and Ta,9 ' 10 W 11 and Re 12 analogs of 1 have been synthesized
TMS
[N3N]MoC1 + Mg
5 C2H4
5C2H4
THF
TMSN
TMS
N
Mo-
/
N
MNJ
1
(1)
in our group. [Et3Si-N 3 N]Ta(C2H4)1 0 has been shown to be thermally unstable and undergoes
decomposition via a unique pathway involving intramolecular abstraction of a proton a to the
equatorial nitrogen in the ligand methylene backbone. 1 is thermally quite stable; toluene-d8
solutions of 1 remain unchanged after heating under vacuum at 60 0C for 14 h. However, when a
sample of 1 is heated to 90 °C in toluene under 1 atmosphere of dinitrogen for a period of 1 week,
the 1H NMR spectrum reveals the presence of [N3N]Mo-N=N-Mo[N 3 N] and [N3N]Mo(N 2 ) along
References begin on page 137
108
Chapter3
with unreacted 1 (equation 2). Presumably, the ethylene ligand is first replaced with dinitrogen to
form [N3 N]Mo(N 2 ) which upon further heating is converted to [N3 N]Mo-N=N-Mo[N 3 N] (see
Chapter 1). The broadness of the TMS resonance of 1 and its coincidental overlap with the TMS
resonance of [N3 N]Mo-N=N-Mo[N3N] precluded an estimate of the extent of decomposition. 1
decomposes rapidly in the solid state when exposed to high vacuum as evidenced by the formation
of a black, oily solid. We speculate that loss of ethylene is the first step in this decomposition
although no products of the reaction have been identified.
90 °C,tol
1
N2
1 + [N3N]Mo(N 2) + {[N 3N]Mo}2(i-N 2 )
(2)
1 is oxidized by ferrocenium triflate to give known [N3 N]MoOTf 8 as the only identifiable
product, according to 1H NMR spectroscopy (equation 3). Upon oxidation, it appears that
backbonding from the cationic d2 metal center is weak and the ethylene ligand is lost with the
resulting formation of [N3 N]MoOTf. In contrast, { [N3NF]W(C 2 H4 ) }OTfl 1 is isolable (the
analogous molybdenum complex has not been synthesized). Presumably, backbonding from the
reducing tungsten center to the ethylene ligand is more efficient in this complex, despite the
presence of the more electron-withdrawing [N3NF] 3- ligand.
TMS
TMS
MTMS
MSTMSTMS
" Mo-N
NO
KVOO-
(3)
FcOTf
N
C2H4
N
Mo-
N
1
SQUID 13 magnetic susceptibility studies have been carried out on solid 1 and a plot of the
molar magnetic susceptibility versus temperature is shown in Figure 3.1. The data can be fit to the
References begin on page 137
109
Chapter3
Curie law over the temperature range 5-300 K to give g = 1.73(1) gB (R = 0.9999), consistent
with 1 being a low-spin Mo(III) complex. It should be noted that in trigonal bipyramidal
complexes of the type [N3N]Mo(L) the lowest lying orbitals are the degenerate dxz/dyz pair. Three
electrons occupy these two orbitals giving rise to a single unpaired spin.
Figure 3.1. Plot of Xm (corrected for diamagnetism using Pascal's constants) versus T for
[N3 N]Mo(C 2 H4 ) (1).
0.08.....................-....
0.07
0.06.
0.05
0
0
o
0.02
0.01
-.
0
50
0
0000000p
i
0
.
..
100
150
200
oin 0 i
250
300
I
350
T (K)
Synthesis and Reactivity of [N3N]Mo(CO)
Attempts to synthesize [N3 N]Mo(CO) (2) directly from [N3 N]MoCl have been
unsuccessful as no reaction is observed between [N3 N]MoCl and magnesium powder in the
presence of one equivalent of CO and [N3 N]MoCl is recovered. A preassociation of CO with
[N3 N]MoCl that might prevent reduction from occurring is ruled out on the basis that resonances
in the 1H NMR spectrum of [N3 N]MoCl under 1 atmosphere of CO are not shifted relative to those
in the 1 H NMR spectrum of [N3 N]MoCl under 1 atmosphere of dinitrogen.
In contrast,
[N 3 N]MoOTf is reduced by magnesium in the presence of CO but the product is not
References begin on page 137
110
Chapter3
[N3 N]Mo(CO) but rather the dimeric complex {(TMSNCH 2 CH 2 )2 N(CH2 CH 2 N)Mo }2 14 formed
by formal loss of TMSOTf. Similar results are obtained when [N3 N]MoOTf is reduced by
magnesium under dinitrogen (see Chapter 1). It should also be noted that [N3NF]WOTf can be
reduced by sodium amalgam in the presence of carbon monoxide to yield [N3NFIW(CO) but
[N3NF]WCl does not react under the same conditions."
However, [N3 NF]W(CNtBu) can be
synthesized by reduction of [N3NF]WOTf or [N3NFIWCI in the presence of tBuNC. 11 The
results described above are puzzling and no satisfactory explanation of them has been arrived at to
date.
Complex 2 can be accessed through the ligand exchange reaction depicted in equation 4.
Exposure of benzene solutions of [N3 N]Mo(N 2 ) to one equivalent of CO results in a color change
from orange to emerald green over the course of 15 min and 2 can be isolated from the reaction as
green needles in 85% yield. If an excess of CO is used in this reaction a mixture of 2 and
[N3 N]MoCOTMS (3, see below) is formed. Complex 3 presumably arises from intermolecular
migration of a TMS group and the formation of 3 serves to further illustrate the susceptibility of the
[N3 N] 3- ligand to degradation via Si-N bond cleavage (see Chapter 1).
O
TMTMS
[N 3N]Mo(N 2 )
1 CO, C6H6
-
TMS
9N,
I
Mo-N
/
- N2
2
(4)
The 1H NMR spectrum of 2 consists of two broad resonances at 13.17 and -38.04 ppm for
the methylene protons of the ligand backbone and a sharper resonance at -2.03 ppm which is
assigned to the TMS groups of the ligand. This spectrum is reminiscent of the 1 H NMR spectrum
of [N3N]Mo(N2) (see Chapter 1) and the observation of a high field and a low field resonance for
References begin on page 137
111
Chapter3
the methylene protons with the TMS resonance falling between them is characteristic of
[N3 N]Mo(L) complexes with the exception of 1 (see above). The IR spectrum of 2 in pentane has
a strong, sharp C-O stretch at 1859 cm- 1 which lies on the low end of the range of typical
stretching frequencies for neutral, terminal CO complexes, 15 indicating that there is considerable
backbonding from the metal into the n* orbitals of the CO ligand. The IR spectrum of 2 in Nujol
exhibits two strong absorptions at 1841 and 1832 cm- 1 . A similar trend of one absorption being
observed in solution and two absorptions observed in the solid state is seen in the IR spectra of
[N3 N]Mo(N 2 ) and in that case we attribute the two absorptions in the solid state spectrum to the
presence of two molecules in the unit cell (see Chapter 1).
Efforts to reduce [N3 N]WC1 have been unsuccessful but [N3 N]W(CO) has been prepared
in low yield by exposure of [N3 N]WCl or [N3 N]WI to excess carbon monoxide. 16 A comparison
of the position of the CO stretches in the IR spectra of 2, [N3 N]W(CO) and [N3NFIW(CO) 1 1 is
instructive. The C-O stretch for [N3N]W(CO) in Nujol is found at 1789 cm- 1 , 40-50 cm- 1 lower
than the corresponding stretch for 2, consistent with the more reducing nature of tungsten
compared to molybdenum. The IR spectrum of [N3 NF]W(CO) in Nujol reveals a C-O stretch at
1846 cm- 1 which is almost identical to the position of that of 2 suggesting that the extent of
backbonding is similar in these complexes and that substitution of W for Mo is offset by the more
electron-withdrawing nature of the [N3NF] 3- ligand.
Heating C6 D6 solutions of 2 at 80 OC for one week results in a slight darkening of the color
of the solution.
1H
NMR spectroscopy reveals the presence of -10%
[N3 N]MoCOTMS (3),
reinforcing Si-N bond cleavage as a common decomposition route for complexes containing the
[N3 N] 3- ligand.
SQUID magnetic susceptibility data for solid 2 are plotted versus temperature in Figure
3.2. The data reveal that 2 behaves as a Curie paramagnet over the temperature range 5-300 K
and fitting the data to the Curie law yields g = 1.74(1) JB (R = 0.9998), a value that is close to the
spin-only moment for a system containing one unpaired electron.
References begin on page 137
112
Chapter3
Figure 3.2. Plot of Xm (corrected for diamagnetism using Pascal's constants) versus T for
[N3N]Mo(CO) (2).
0.08
..............
...................................
.....
0.07.
0.06
.... ...
.
_...
...
....... .........
.
Z .
..
0
0.05
0.04 0
50
150
200
250
300
350
T (K)
When THF solutions of 2 are stirred over magnesium powder in the presence of TMSC1 a
color change from deep green to yellow is observed over the course of fifteen minutes. The
diamagnetic oxycarbyne complex [N3N]MoCOTMS (3) can be isolated from the reaction as pale
yellow needles in 91% yield (equation 5). In the absence of TMSC1, two other diamagnetic
complexes are observed by 1H NMR spectroscopy and are tentatively formulated as
{[N 3 N]Mo(CO) }2Mg(THF)2 and {[N 3 N]Mo(CO) }MgCl(THF) 2 , the CO analogs of
{[N3 N]Mo(N 2 )12 Mg(THF) 2 and {[N3 N]Mo(N 2 )}MgCI(THF) 2 , respectively (see Chapter 1). No
attempt has been made to isolate these complexes. The 1H NMR spectrum of 3 consists of two
TMS resonances and a pair of triplets for the methylene protons on the ligand backbone. The
13 C
NMR spectrum of 3 also reveals TMS groups in two environments and the resonance for the
alkylidyne carbon is found at 208.3 ppm. The IR spectrum of 3 taken in Nujol does not have a
readily assignable C-O stretch. The formation of 3 is perhaps not surprising in light of the
documented propensity for tungsten and molybdenum triamidoamine complexes to form strong M-
References begin on page 137
113
Chapter3
E bonds (E = CR, N, P, As) 1 and analogs of 3, namely [N 3 N]WCOTMS
6
and
[N3 NF]WCOTMS," have been synthesized in our laboratory. In general though, siloxycarbyne
OTMS
[N3N]Mo(CO)
Mg, TMSC
THF
TMS
TMM
t
I
N
3
(5)
17
complexes are rare, although examples such as M(CO)(COSiR 3 )(DMPE) 2 (M= V, Ta,18,19
Nb 18 ) have been isolated as intermediates in the reductive coupling of CO to form acetylene
diethers at V, Ta and Nb metal centers.
Alkyl- and Arylisocyanide Complexes
As with carbon monoxide, no reaction is observed between [N3 N]MoCl and magnesium
powder in the presence of one equivalent of tBuNC. However, paramagnetic [N3N]Mo(CNtBu)
(4) can be isolated from the reduction of [N3 N]MoC1 by Na/Hg amalgam in the presence of
tBuNC but the reaction is not clean and 4 is contaminated with [N3 N]MoC1. A clean, highyielding route to 4 was found in the reaction of [N3 N]Mo(N 2 ) with tBuNC (equation 6). Upon
addition of tBuNC to toluene solutions of [N3 N]Mo(N 2 ) a color change to deep orange is
discernible and 4 is isolated from the reaction as rust-colored needles in 96% yield. The 1H NMR
spectrum of 4 exhibits broad resonances at 13.37 and -39.00 ppm for the methylene protons of the
TREN ligand, the resonances for the tBu and TMS groups being observed at 3.76 and 0.12 ppm,
respectively. The IR spectrum of 4 in Nujol has a broad absorption at 1838 cm-1 (free tBuNC =
2143 cm- 1). This value should be compared with that of [N3 NF]W(CNtBu) (1684 cm-1), the
crystal structure of which revealed that the isocyanide ligand is quite bent (C-N-C = 132.2(10)). 11
References begin on page 137
114
Chapter3
This result and the low C-N stretching frequency were attributed to extensive r backbonding from
the metal center to the isocyanide ligand and it was suggested that [N3 NF]W(CNtBu) is best
formulated as a W(V) imido carbene complex. The higher C-N stretching frequency of 4 suggests
that the degree of 7t backbonding is less in 4 and that it contains a linear isocyanide ligand and so is
best viewed as a Mo(ll) isocyanide complex.
tBu
TMS
i
1 tBuNC, tol
[N3 N]Mo(N2 )
N2
-NN2
MS
NU
N-
I
-N/
4
(6)
SQUID magnetic susceptibility measurements on solid 4 have been carried out and a plot of
the molar magnetic susceptibility versus temperature is shown in Figure 3.3. Like 1 and 2, 4
behaves as a Curie paramagnet over the temperature range 5-300 K and the data can be fit to the
Curie law yielding g = 1.74(1) gB, consistent with 4 being a low spin d3 complex with one
unpaired electron.
4 reacts smoothly with ferrocenium triflate in THF to give {[N3N]Mo(CNtBu) }OTf (5)
quantitatively as a burnt-orange powder. The 1H NMR spectrum of 5 is typical of d2 [N3 N]Mo
complexes, the resonances for the methylene protons of the ligand backbone (-29.38 and -98.14
ppm) occurring upfield of the resonance attributed to the TMS groups of the ligand (12.83 ppm).
The C-N stretching frequency of 5 in THF appears at 2147 cm- 1 , reflecting the weak r
backbonding from the cationic d2 metal center compared to that of the d3 metal center of 4. Efforts
to crystallize 5 have been unsuccessful and so satisfactory elemental analyses have not been
obtained.
References begin on page 137
115
Chapter3
SQUID magnetic susceptibility measurements on 5 over the temperature range 5-300 K
reveal a behavior analogous to that observed for [N3N]MoMe and [N3 N]MoCI (X approaches a
constant as T approaches 0 K),8 and which we now assume to be characteristic of paramagnetic d2
[N3N]Mo complexes of this general type (Figure 3.4). The data can be fit to the Curie-Weiss law
(X = pL2/8(T-0)) over the temperature range 30-300 K (g = 2.71(4) 9B, 0 = -5(1) K), consistent
with two unpaired electrons being present. In C3-symmetric triamidoamine complexes the frontier
7c orbitals (dxz and dyz) are strictly degenerate and in 5 we assume these orbitals are singlyoccupied giving rise to two unpaired spins in the molecule.
In contrast to 5,
{[N3NF]W(CNtBu) OTf 1 is diamagnetic suggesting that backbonding from tungsten into the 7c*
orbitals of the isocyanide ligand is sufficient to break the degeneracy of the dxz/dyz orbitals.
4 does not decompose in solution when stored at room temperature for a period of days
according to 1 H NMR spectroscopy. However, 4 proved to be thermally unstable at elevated
temperatures and upon heating toluene solutions of 4 to 86 'C for 36 h, a color change from orange
to yellow is observed. If the thermolysis is carried out in C6 D6 in a sealed NMR tube, 1 H NMR
spectra of the crude reaction mixture reveal three broad, paramagnetically-shifted resonances at
7.73, -25.7 and -112.4 ppm suggesting formation of a new C3-symmetric complex. Resonances
at 4.80, 1.61 and 0.95 ppm are also observed and are assigned to isobutylene, isobutane and
hexamethylethane, products arising from the disproportionation and coupling of tBu radicals.
[N3 N]MoCN (6) can be isolated from the reaction as a yellow, crystalline solid in 88% yield. A
C-N stretch could not be located in the IR spectrum of 6. It should be noted that the IR spectrum
of the analogous tungsten complex, [N3 N]WCN, 16 also lacks an absorption in the region 22002000 cm 1 .
The reaction depicted in equation 7, that is, formation of a cyanide complex via dealkylation
of an isocyanide complex has some precedent in the literature. Upon refluxing in ethanol,
[(tBuNC) 7 Mo] 2 + loses a carbonium ion to form [(tBuNC) 6 Mo(CN)]+, although the organic
products of the reaction were not isolated. 20 Reaction of [(tBuNC) 7 Mo] 2 + with zinc in THF
yields [(tBuNC) 4 Mo(tBuHNCCNHtBu)(CN)]+, a product in which both reductive coupling and
References begin on page 137
116
Chapter 3
Figure 3.3. Plot of Xm (corrected for diamagnetism using Pascal's constants) versus T for
[N3 N]Mo(CNtBu) (4).
0.08
0.07
0.06
0.05
m 0.04
O
0.03
0
0.02
0.01
0
0
50
100
150
200
250
300
350
T (K)
Figure 3.4. Plot of Xm (corrected for diamagnetism using Pascal's constants) versus T for
{ [N3N]Mo(CNtBu) }OTf (5).
0.04
0.035
0.03
0.025
X m 0.02
0.015
0.01
0.005
0
0
50
100
150
200
250
300
350
T (K)
References begin on page 137
117
Chapter3
dealkylation of the isocyanide ligands has occurred.2 1 Perhaps the most closely related example to
6 is that of trans-Mo(CN)2(Me8[16]aneS4) (Me8[16]aneS4 = 3,3,7,7,11,11,15,15,-octamethyl1,5,9,13-tetrathiacyclohexadecane). 2 2 The precursor complex, Mo(CN)(tBuNC)(Me 8[ 16]aneS 4 )
is formed by a ligand exchange reaction from the bis-dinitrogen complex and it decomposes even at
-30 'C to the dicyanide complex.
TMSI
86 OC, tol
[N3N]Mo(CN t Bu)
86 C, t
TMS
Mo
StBu MN
N
N
6
(7)
Single crystals of 6 were grown from saturated diethyl ether solutions at -30 'C and
examined in an X-ray study; a half a molecule of diethyl ether was found in the unit cell.
Crystallographic data and collection and refinement parameters are given in Table 3.1. The
molecular structure of 6 along with the atom-labeling scheme is shown in Figure 3.5 while selected
bond lengths, bond angles and dihedral angles are listed in Table 3.2. As expected, the structure
of 6 bears a striking resemblance to that of [N3 N]MoC1. 2 3 The Mo-Namide bond distances are
statistically identical in the two complexes, as are the Mo-Nax bond distances.
In 6 and
[N3 N]MoCl the TMS groups are all oriented upright with the Nax-Mo-Neq-Si dihedral angles close
to 1800, indicative of little steric pressure within the pocket. For comparison, in [N3 N]MoOTf the
dihedral angles range from 136-1430 as the TMS groups twist in response to the presence of the
sterically-bulky triflate ligand within the pocket. 8 The Mo-C(7) and C(7)-N(5) bond lengths of
2.182 and 1.113
A
in 6 are close to the corresponding bond lengths in trans-
Mo(CN) 2 (Me8[16]aneS4) (2.219(7) and 1.086(10) A, respectively). 22
References begin on page 137
118
Chapter3
Figure 3.5. A view of the structure of [N3 N]MoCN (6).
N(5)
Q
C(7)
Si(2)
Si(1)
Si(3)
N(1)
N(4)
A plot of the molar magnetic susceptibility of 6 versus temperature is shown in Figure 3.6
and is characteristic of paramagnetic d2 [N3 N]MoX complexes of this general type (see 5 above).
The data can be fit to the Curie-Weiss law (x = t 2/8(T-0)) over the temperature range 30-300 K to
give g = 2.73(1)
B and 0 = -7.1(3). A plot of JReff versus temperature for 6 is shown in Figure
3.7 and illustrates how Iteff decreases rapidly below 50 K. Similar behavior has been observed for
[N3 N]MoC18 and is attributed to a combination of spin-orbit coupling and low-symmetry ligand
field components that result in zero field splitting of the d2 ground-state spin triplet.24
References begin on page 137
119
Chapter3
Table 3.1. Crystallographic data, collection parameters and refinement parameters for 6 and 8.
8
Empirical Formula
C18H44 MoN 5 0 0 .50 Si3
C 15 H38 MoN 4 Si3
Formula Weight
518.79
454.70
Diffractometer
SMART/CCD
SMART/CCD
Crystal Dimensions (mm)
0.37 x 0.32 x 0.23
0.60 x 0.43 x 0.34
Crystal System
Tetragonal
Monoclinic
Space Group
P42 1m
P2 1/n
16.5384(4)
8.972(3)
b (A)
16.5384(4)
17.308(4)
c (A)
9.8908(3)
15.398(3)
90
90
90
100.61(3)
90
90
V (A3), Z
2705.32(12), 4
2350.2(11), 4
Dcale (Mg/m 3)
1.274
1.285
Absorption coefficient (mm-rl)
0.633
0.716
Fo00
1100
960
Temperature (K)
183(2)
188(2)
O range for data collection (0)
1.74 to 23.23
1.79 to 23.26
Reflections collected
11177
9331
Unique Reflections
2052
3337
R
0.0292
0.0231
Rw
0.0320
0.0242
GoF
0.936
1.125
P(0)
References begin on page 137
120
Chapter3
Table 3.2. Selected bond lengths and bond angles for [N3 N]MoCN (6).
Bond Lengths (A)
Mo-C(7)
2.182(6)
Mo-N(1)
1.980(5)
Mo-N(2)
1.970(3)
Mo-N(3)
1.970(3)
Mo-N(4)
2.210(5)
C(7)-N(5)
1.113(8)
Bond Angles (deg)
Mo-C(7)-N(5)
179.8(6)
C(7)-Mo-N(4)
179.1(2)
N(1)-Mo-N(2)
118.54(11)
N(1)-Mo-N(3)
118.53(11)
Si(1)-N(1)-Mo
128.1(2)
Si(2)-N(2)-Mo
128.3(3)
N(1)-Mo-N(4)
80.9(2)
N(2)-Mo-N(4)
80.92(12)
Dihedral Angles (deg)a
N(4)-Mo-N(3)-Si(3)
179.34
N(4)-Mo-N(2)-Si(2)
-179.55
N(4)-Mo-N(1)-Si(1)
-180.00
N(4)-Mo-C(7)-N(5)
0.00
aObtained from a Chem-3D Drawing
References begin on page 137
121
Chapter3
Figure 3.6. Plot of Xm (corrected for diamagnetism using Pascal's constants) versus T for
[N3 N]MoCN (6).
0 .04
m
.................
-....................................................
. .............. ................. ..................
0.035
...
0.035
0.025
0 S.
...
o
.
.
.
0.02
0.015
0.01
0.005
0
50
0
150
100
200
250
300
350
T (K)
Figure 3.7. Plot of Reff versus T for [N3 N]MoCN (6).
2.8
2.6
2.4
---o
2.2
o
eff
1.8
-
i
1
-
--
-
-
-
-
...
......
.---
I
i
o......
.................. ........ .................. ........
0l
1.6
1.4
1.2
0
References begin on page 137
50
100
150
200
250
300
350
T (K)
122
Chapter3
Reaction of [N3N]Mo(N 2 ) with ArNC (Ar = 2,6-Me 2 C 6 H3) yields [N3 N]MoCNAr (7) in
good yield as deep red plates. Unlike 4, 7 is thermally stable and C6 D6 solutions of 7 show no
evidence for decomposition when heated to 80 'C for 12 h. The C-N stretch for 7 is found at 1740
cm- 1 and lies between that of the linear (1959 cm-1) and bent (1658 cm - 1 ) PhNC ligands of transMo(PhNC) 2 (Me 8 [16]aneS4). 22
Attempted Synthesis of Other [N3 N]Mo(III) Complexes
Attempts to synthesize [N3N]Mo(L) complexes where L is a a donor such as a phosphine
or nitrile have been unsuccessful. [N3 N]MoCl is reduced by magnesium in the presence of two
equivalents of PMe 3. If the reaction is carried out under dinitrogen, 1H NMR spectroscopy reveals
the presence of two diamagnetic complexes whose resonances are consistent with their formulation
as {[N3 N]Mo(N 2 )}2 Mg(PMe3)2 and {[N3 N]Mo(N 2 ) }MgCl(PMe3) 2 . Carrying out the reaction in
the absence of dinitrogen yields [bitN 3 N]Mo (8, see below) and [N3 N]MoH as the only
identifiable species according the 1 H NMR spectroscopy. It is abundantly clear from these
observations that PMe 3 does not inhibit reduction (unlike CO and tBuNC).
Arguably,
[N 3 N]Mo(PMe 3 ) may not be accessible on steric grounds and reactions with other phosphines
have not been attempted. However, attempts to synthesize [N3NF]W(PMe3) have also been
unsuccessful despite the larger bowl-like pocket of these complexes. 11 It appears that dx-pt
backbonding is an important component of the bonding picture in [N3 N]Mo(L) complexes and is
inherent to their stability. If [N3 N]Mo(PMe3) is generated in situ, the weak it acceptor ability of
PMe 3 would render this ligand extremely labile and easily replaced by dinitrogen.
[N3 N]Mo(NCCH 3 ) has also proved inaccessible. [N3N]MoCl is not reduced by magnesium in
THF in the presence of two equivalents of acetonitrile. Similarly, no reaction is observed when
acetonitrile is used as the solvent and in both cases [N3 N]MoCl is recovered. Furthermore, the
dinitrogen ligand in [N3 N]Mo(N 2 ) does not undergo exchange with CH 3 CN.
Since
[N3 N]W(PhNNPh)(H) has been isolated, 6 synthesis of [N3N]Mo(PhNNPh) seemed a reasonable
References begin on page 137
123
Chapter3
proposition on steric grounds. Unfortunately, once again our efforts were thwarted by the inability
of magnesium to reduce [N3 N]MoCl in the presence of azobenzene.
Synthesis and Reactivity of [bitN3 N]Mo
In an attempt to isolate the elusive trigonal monopyramidal complex [N3N]Mo, we began to
explore the reduction of [N3 N]MoCI in the absence of donor ligands. Reaction of [N3N]MoCl
with magnesium powder in an evacuated vessel over the course of seven days leads to a color
change from orange to blood-red.
1H
NMR spectra of the crude reaction mixture taken under
dinitrogen reveal the presence of two paramagnetic products, one of which is readily identified as
the known hydride complex, [N3 N]MoH. 8 The second product, [bitN 3 N]Mo (8) exhibits eight
resonances between +19 ppm and -126 ppm and has apparent mirror symmetry (equation 8). 8
can be separated from [N 3 N]MoH by recrystallization from hexamethyldisiloxane and is isolated as
a blood-red, crystalline solid in good yield.
H
TMS
TMS
Mg, THF
L3
O
7 days
TMSN
CH
SiMe
M-
+
N
8
(8)
The molecular structure of 8 was confirmed by an X-ray study. Single crystals of 8 were
grown from saturated hexamethyldisiloxane solutions at -20 °C. Crystallographic data, collection
parameters, and refinement parameters for 8 are given in Table 3.1. The molecular structure of 8
(two views) along with the atom-labeling scheme are shown in Figure 3.8 while selected bond
lengths, bond angles and dihedral angles are given in Table 3.3. The Mo-C(11) bond is somewhat
longer than the Mo-C bonds in [N3 N]MoMe 8 (2.188
References begin on page 137
A) and
[N3N]Mo(cyclohexyl) 8 (2.167 A) but
124
I
--
----
---------
- --
-
- --
----- Ili~- -'.'-.
Chapter3
Figure 3.8. Two views of the structure of [bitN 3 N]Mo (8).
C(11)
N(3)
References begin on page 137
125
..-Ir
Chapter3
Table 3.3. Selected bond lengths and bond angles for 8.
Bond Lengths (A)
Mo-N(1)
1.948(2)
Mo-N(2)
1.983(2)
Mo-N(3)
1.994(2)
Mo-N(4)
2.237(2)
Mo-C(11)
2.249(3)
N(1)-Si(1)
1.734(2)
N(2)-Si(2)
1.743(2)
N(3)-Si(3)
1.743(2)
C(11)-Si(1)
1.849(3)
C(22)-Si(2)
1.873(3)
C(33)-Si(3)
1.869(3)
Bond Angles (deg)
Mo-N(1)-Si(1)
102.30(10)
Mo-N(2)-Si(2)
Mo-N(3)-Si(3)
125.87(11)
N(1)-Si(1)-C(1 1)
92.93(11)
N(2)-Si(2)-C(22)
108.99(11)
N(3)-Si(3)-C(33)
110.56(12)
Mo-C(11)-Si(1)
88.35(10)
127.08(10)
N(1)-Mo-C(11)
76.13(9)
N(4)-Mo-C(11)
155.16(8)
N(1)-Mo-N(2)
116.95(8)
N(1)-Mo-N(3)
118.27(8)
N(2)-Mo-N(3)
116.83(8)
N(1)-Mo-N(4)
79.03(8)
N(2)-Mo-N(4)
81.40(8)
N(3)-Mo-N(4)
81.20(7)
Dihedral Angles (deg)a
N(4)-Mo-N(1)-Si(1)
176.08
N(4)-Mo-N(2)-Si(2)
178.86
N(4)-Mo-N(3)-Si(3)
170.94
Mo-N(1)-Si(1)-C(11)
-3.99
aObtained from a Chem-3D Drawing
References begin on page 137
126
Chapter3
the Mo-Namide bond distances are comparable to those found in 6 and the Na-Mo-Neq-Si dihedral
angles are all close to 1800 consistent with little steric strain in the molecule. The Mo atom lies
0.325 A out of the plane defined by the amide nitrogens in the direction of C(11) and the fourmembered ring is almost planar (Mo-N(1)-Si(1)-C(11) = -40). The absence of distortion in the
MoN4 core is somewhat surprising in view of the presence of the Mo-C-Si-N ring and relatively
small N(4)-Mo-C(11) angle (155.16(8)0). 8 has approximate mirror symmetry in the solid state as
highlighted by the view down the Mo-N(4) axis and is in complete accord with the NMR data.
Since resonances for protons on carbons bound directly to Mo are not observed for paramagnetic
[N3N]Mo(alkyl) complexes, presumably due to their proximity to the paramagnetic center, eight
resonances would be expected and are observed in the 1 H NMR spectrum of 8. Although C-H
activation in TMS amido complexes is relatively well-known, 25-28 X-ray structures of complexes
that contain a MNSiC ring are rare.29 -31 One such species is Zr[CH 2 SiMe 2 N(SiMe 3 )]2(dmpe) 29
which contains two planar, four-membered metallacyclic rings analogous to that found in 8 (C-ZrN = 760; Zr-N-Si = 970; N-Si-C = 990; Si-C-Zr = 870).
A plot of the molar magnetic susceptibility of 8 versus temperature is shown in Figure 3.9
and is similar to other d 2 complexes (see 5 and 6 above). Fitting the data to the Curie-Weiss law
(X = g2/8(T-0)) over the temperature range 30-300 K yields R = 2.84(1) and 2.53(1) RB and 0 =
-4.1(4) and -3.4(3) K (2 runs).
8 is thermally stable and 1H NMR spectra of toluene-d8 solutions of 8 show no evidence
for decomposition when heated to 90 oC under dinitrogen or under vacuum for one week.
Andersen has demonstrated that related thorium and uranium metallacycles undergo insertion
reactions with CO and tBuNC to yield five-membered metallacyclic complexes resulting from
formal insertion of tBuNC and CO into a silicon-carbon bond. 32 The dimeric metallacyclic
complex, {[(Me 3 Si)2 N]V[p.-CH 2 SiMe 2 N(SiMe3)] }2 also reacts with CO in a similar manner 30 and
so an investigation of addition/insertion reactions of 8 was undertaken. However, 8 does not react
with ethylene or CO but exposure of THF solutions of 8 to D2 (1 atm) results in a color change
from blood-red to yellow over a period of 2 days. [dl-N 3N]MoD (9) is formed quantitatively
References begin on page 137
127
Chapter3
Table 3.4. Selected characterization data for paramagnetic [N3N]Mo complexes.
Complex
% Yield
Morphology
I.so
geff
[N3 N]MoC 2 H4 (1)
97
purple needles
1.73
1.73
[N3 N]MoCO (2)
85
green needles
1.73
1.77
[N3 N]MoCNtBu (4)
96
rust needles
1.73
1.74
{[N3 N]MoCNtBu}OTf (5)
92
orange powder
2.83
2.71
[N3 N]MoCN (6)
88
yellow cubes
2.83
2.73
[N3 N]MoCNAr (7)
62
red plates
1.73
na
[bitN 3N]Mo (8)
72
red cubes
2.83
2.87
[dl-N 3 N]MoD (9)
100
yellow needles
2.83
2.83
Figure 3.9. Plot of Xm (corrected for diamagnetism using Pascal's constants) versus T for
[bitN 3 N]Mo (8).
................. ................. ..................
............... ................. ..................
0 .0 5
0.03
.02 . ..........
00.0
................
................. ..................
................. .................
................. .................. ................. .................
0.0
.. .......
0.01
0
50
......
100
150
200
250
300
350
T (K)
References begin on page 137
128
Chapter3
according to 1H NMR spectroscopy and the 2 H NMR spectrum of 9 in benzene reveals a singlet at
16.48 ppm indicating that the second deuterium is located in a TMS group of the ligand (equation
9). The reversibility of the cyclometallation reaction has been confirmed by Dr. Scott Seidel and
upon heating toluene solutions of [N3 N]MoH to 105 'C formation of 8 and dihydrogen is
observed.8 SQUID magnetic susceptibility data for 9 can be fit to the Curie-Weiss law over the
temperature range 30-300 K yielding g = 2.87(3) lg and 08 = -0.2(6).
TMS
TMS
TMS.N
+ D2
TMS
D
N
(9)
...
o,,
I
NO' Mo-N
-D 2
8 reacts rapidly with tBuNC and upon addition of tBuNC to a toluene solution of 8 an
immediate and dramatic color change to deep green is evident. This green color fades within
minutes to give a clear, orange solution. The product (10) can be isolated by crystallization from
hexamethyldisiloxane as orange, diamagnetic crystals (equation 10, TMS groups of 10 omitted for
clarity). The 1H NMR spectrum of 10 in C6 D6 has multiple resonances for the methylene protons
I
N
TMS
t-Bu
(10)
C-
CH 2
t-Bu
/
2 tBuNC
CH2
N
IN
SiMe 2
Mo -- N
of the ligand backbone and a pair of singlets at 1.50 and 1.45 ppm integrate for 18H, consistent
with incorporation of two equivalents of tBuNC. A single resonance is observed for the TMS
References begin on page 137
129
Chapter3
groups of the ligand and the presence of a plane of symmetry in 10 is confirmed by the
13 C
NMR
spectrum which exhibits four resonances for the methylene carbons of the ligand backbone. The
13 C
NMR spectrum also reveals quaternary carbon resonances at 247.4 and 167.2 ppm. The IR
spectrum of 10 has a strong absorption at 1586 cm- 1 , characteristic of a C-N double bond stretch.
On the basis of the available data, 10 is tentatively formulated as an r 2 -iminoacyl imine complex as
shown in equation 10, the downfield shifts in the
13 C
NMR spectrum of the quaternary carbons
being characteristic of the iminoacyl and imine functionalities (247.4 and 167.2 ppm respectively).
Insertion of isocyanides into metal-carbon bonds 3 3 -35 and coupling of isocyanides at metal
centers 36 -3 8 are well-documented reactions. Futhermore, Zr 39 and Hf4 0 metallacyclobutane
complexes have been shown to undergo double insertion/coupling reactions with tBuNC,
analogous to that proposed for 8, yielding related rT2 -iminoacyl imine complexes.
DISCUSSION
The central theme of the chemistry discussed in this chapter is the use of [N3 N]Mo(N 2 ) to
synthesize organometallic complexes of molybdenum in the relatively rare oxidation state of 3+ via
ligand exchange reactions. However, we have not determined whether these reactions are
proceeding via an associative or dissociative mechanism. In general, [N3N]Mo(L) complexes
cannot be synthesized directly by reduction of [N3N]MoCl in the presence of the appropriate ligand
for reasons that are not entirely clear. [N3 N]Mo(L) complexes show a tendency to be oxidized to
Mo(IV) complexes either by treatment with an oxidant or, as in the case of [N3 N]Mo(CNtBu) (4),
by expulsion of an organic radical. In the case of [N3 N]Mo(C 2 H4 ) (1), the ethylene ligand is lost
upon oxidation consistent with the bonding picture in [N3 N]Mo(C 2 H4 ) being closer to the DewarChatt model rather than the metallacyclopropane model.
In general, comparison of the reactivity of [N3 N]Mo(L) complexes with that of the
analogous [N3NF]Mo(L) complexes has not been possible, as the organometallic chemistry of
such complexes is relatively unexplored. However, comparisons with [N3NF]W(L) complexes
have been made where possible and reveal striking differences between the two systems. For
References begin on page 137
130
Chapter3
example, both {[N3 NF]W(C2H4) }OTf 11 and {[N3 NF]W(CNtBu) }OTf 11 are diamagnetic whereas
{ [N3 N]Mo(CNtBu) }OTf (5) is paramagnetic. The diamagnetism of the [N3NFIW complexes
might be explained by an increase in dx-pt backbonding as would be expected for tungsten relative
to molybdenum i.e."oxidation" to W(VI). Alternatively, it may indicate coordination of triflate in
these complexes. Such coordination of triflate would break the degeneracy of the dxz/dyz orbitals,
accounting for the observed diamagnetism. In contrast, coordination of triflate in 5 is unlikely due
to steric congestion in the apical pocket as a result of the bulky TMS groups on the ligand.
An interesting difference between [N3 N]Mo and [N3 N]W complexes has also been
uncovered. As evidenced by the mode of synthesis, [N3 N]MoCN (6) is thermally stable, in stark
contrast to the behavior of [N3 N]WCN which decomposes at room temperature yielding two
diamagnetic products. 16 Neither of these products was fully characterized but it was proposed that
one arose from the intermolecular coupling of cyanide ligands by analogy to the coupling of
acetylides in [N3 N]Mo complexes. 4 1 In light of the X-ray structure and paramagnetism of
[N 3 N]MoCN we are confident in our formulation of [N3 N]MoCN as a monomeric cyanide
complex.
The mechanism by which [bitN 3 N]Mo (8) might be formed deserves some comment. We
propose that [N3 N]MoCl is reduced by magnesium to {[N3 N]MoC I)- which loses Cl- to form the
trigonal monopyramidal species A (equation 11). Oxidative addition of a C-H bond of a TMS
group of the ligand to the metal center, possibly in a reversible manner, yields the Mo(V) species
B. Loss of a hydrogen radical then produces 8. The fate of the hydrogen radical is uncertain
(equation 12). It may react with A to give [N3N]MoH. Alternatively, coupling of two hydrogen
TMS
TMS
TMS
,
/
Mo-N
1
A
References begin on page 137
TMS
H
CH 2 - SiMe 2
H\11
TMS N."Mo-N
B
/
TMS
-H
TMS N.,
CH 2 - SiMe 2
Mo-N
I
(11)
8
131
Chapter3
radicals would produce dihydrogen which could then react with 8 to give [N3 N]MoH (see
equation 9 above). Either of these scenarios would lower the yield of 8 and 1H NMR spectra of
the crude reaction mixture indicate that [N3 N]MoH and 8 are formed in an approximate ratio of
1:4. The related complex [bitN 3 N]Ti has been synthesized by thermal decomposition of
[N3N]MoH
A
-
H
I H
+H
8
H2
o
[N3N]MoH
(12)
[N3N]Ti(s-Bu) and upon heating [N3 N]Ti(s-Bu) in the presence of dihydrogen a resonance
attributed to the Ti(IV) hydride complex is observed by 1 H NMR spectroscopy. 2 8 Also,
[N3 N]WH decomposes slowly upon heating to 85 °C to give the known trihydride complex,
[N3 N]W(H) 3 and a complex possessing mirror symmetry which was purported to be [bitN 3 N]W
but which was not isolated. 16 These observations confirm that [N3N]MH complexes where M =
Ti, Mo, W are, to varying degrees, susceptible to cyclometallation reactions via C-H activation of a
TMS group of the ligand and that, in some cases, this reaction is reversible.
A major impetus for this work was to determine whether the trigonal monopyramidal
complex [N3N]Mo could be isolated. First row analogs of this complex from titanium to iron have
been synthesized and were found to be high-spin. 2
The related trigonal complex
Mo[N(C(CD 3 )2 CH 3 )(3,5-Me 2C 6 H3 )13 is also isolable and has a high-spin configuration. 4 2 A
detailed investigation of the dinitrogen chemistry of [N3 N]Mo complexes (see Chapter 1) suggests
that a low-spin configuration of [N3 N]Mo is optimal to bind dinitrogen. In such a configuration
the presence of an empty orbital on the metal center would also render [N3 N]Mo susceptible to
oxidative addition reactions which are proposed to account for the formation of [bitN 3 N]Mo (see
above). These results suggest that [N3N]Mo is an unstable species and that its high energy and
reactivity could be a consequence of its low-spin configuration.
Clearly, isolation of a
molybdenum trigonal monopyramidal complex in TREN-based systems will require employment
of a more robust ligand.
References begin on page 137
132
Chapter3
EXPERIMENTAL PROCEDURES
General Details. All experiments were performed under a nitrogen atmosphere in a
Vacuum Atmospheres drybox or by standard Schlenk techniques unless otherwise specified.
Pentane was washed with sulfuric acid / nitric acid (95/5 v/v), sodium bicarbonate, and water,
stored over calcium chloride, and distilled from sodium benzophenone ketyl under nitrogen.
Toluene was distilled from sodium, and CH 2 C12 was distilled from CaH 2 . Anhydrous diethyl
ether and THF were sparged with nitrogen and passed through alumina columns. 4 3
Hexamethyldisiloxane was purchased from Aldrich, dried over sodium and then vacuum
transferred into a small storage flask. All solvents were stored in the dry box over activated 4 A
molecular sieves.
NMR data were obtained at 300 or 500 MHz ( 1H), 75.4 MHz ( 13 C), 46.0 MHz (2 H) and
282 MHz (19 F). Chemical shifts are listed in parts per million downfield from tetramethylsilane
for proton and carbon.
19 F
chemical shifts are listed in parts per million downfield from CFC13 as
an external standard and 2 H NMR spectra were referenced to external C6D6 . Coupling constants
are listed in Hertz. Spectra were obtained at 25 OC unless otherwise noted. Benzene-d6 and
toluene-d8 were pre-dried on CaH2, vacuum transferred onto sodium and benzophenone, stirred
under vacuum for two days and then vacuum transferred into small storage flasks and stored over
molecular sieves. THF-d 8 was dried over sodium and vacuum transferred into a small storage
flask and stored over molecular sieves. [N3 N]MoCl 8 and ferrocenium triflate44 were prepared as
described in the literature. Magnesium powder, tBuNC, ethylene, carbon monoxide and TMSC1
were purchased from commercial vendors and used as received.
Elemental analyses (C, H, N) were performed in our laboratory using a Perkin-Elmer 2400
CHN analyzer or by Microlytics Analytical Laboratories of Deerfield MA. X-ray data were
collected on Siemens SMART/CCD diffractometer and general experimental details are described in
the literature. 45
SQUID Magnetic Susceptibility Measurements. Measurements were carried out
on a Quantum Design SQUID magnetometer. Data were obtained at a field strength of 5000
References begin on page 137
133
Chapter3
Gauss. Straws and gel caps (Gelatin Capsule No. 4 Clear) were purchased from Quantum Design.
The sample was prepared in the drybox by the following method. A gel cap and a square of
parafilm were weighed. The sample was placed in the gel cap and the parafilm inserted above it.
The gel cap was closed and the mass of the sample was ascertained by weighing the loaded gel
cap. The gel cap was placed in a straw which was then mounted on the sample rod and placed in
the magnetometer. Two runs were performed on the sample - one from 5 to 300 K and a second
from 300 to 5 K. Measurements were made at the following increments: 5-10 K (every 1 K), 1020 K (every 2 K), 20-50 K (every 3 K), 50-100 K (every 5 K), 100-200 K (every 10 K), 200-300
K (every 20 K).
[N3 N]MoC 2 H 4 (1). [N3 N]MoCl (650 mg, 1.32 mmol) was dissolved in 15 mL THF
and placed in a glass bomb with a stirring bar and magnesium powder (64 mg, 2.67 mmol). The
vessel was sealed and subjected to three freeze-pump-thaw cycles to remove any dinitrogen
present. Ethylene (-5eqs) was condensed onto the frozen solution. Upon thawing the solution
was stirred for 12 h during which time the color of the solution changed from orange/red to purple.
The solvent was removed in vacuo and the residue extracted with 20 mL pentane. After filtration
through Celite, the pentane was removed in vacuo to give a purple solid. Recrystallization from
hexamethyldisiloxane at -20 'C afforded the product as purple needles; yield 623 mg (97%).
NMR(C 6 D6 ) 8 3.63 (TMS).
1H
t = 1.73 p.B. Anal. Calcd. for C17H4 3N4Si3Mo: C, 42.21; H, 8.96;
N, 11.58. Found: C, 42.39; H, 9.31; N, 11.55.
[N 3 N]Mo(CO) (2). [N3 N]Mo(N 2 ) (100 mg, 0.21 mmol) was dissolved in 5 mL of
benzene and sealed in a 25 mL bomb. The solution was subjected to two freeze-pump-thaw cycles
and 1 equivalent of carbon monoxide was introduced. Over 15 min the solution changed color
from orange-red to emerald green. After 3 h the solvent was removed in vacuo and the residue
extracted with 5 mL of pentane. The pentane solution was cooled to -20 °C to give the product as
green needles; yield 85 mg (85%).
1H
NMR(C 6 D6 ) 8 13.17 (CH 2 ), -2.03 (TMS), -38.04 (CH 2 ).
IR(Nujol, cm - 1) 1841, 1832 (C-O). IR(Pentane, cm - 1) 1859 (C-O). . = 1.77 gpB. Anal. Calcd.
References begin on page 137
134
Chapter3
for C 16 H39 N4 Si 3 MoO: C, 39.73; H, 8.13; N, 11.58. Found: C, 39.54; H, 8.18; N, 11.55.
[N3 N]MoCOTMS (3). [N3 N]MoCO (100 mg, 0.21 mmol) was dissolved in 5 mL of
THF. Magnesium powder (20 mg, 0.87 mmol) and TMSCI (45 gL, 0.36 mmol) were added and
the mixture was stirred for 3.5 h. The solvent was removed in vacuo and the residue extracted
with 7 mL of pentane. The pentane solution was filtered through a pad of Celite and the volume
was reduced to 3 mL. The pentane solution was cooled to -20 OC to give the product as pale
yellow needles; yield 105 mg (91%).
IH NMR(C 6 D 6 ) 5 3.48 (t, NCH 2 CH 2 N), 3.24 (t,
NCH 2 CH 2 N), 0.49 (s, TMS), 0.40 (s, TMS).
13 C
NMR(C 6 D6 ) 8 208.34 (MoCOTMS), 53.53
(NCH 2 CH 2 N), 52.56 (NCH 2 CH 2 N), 5.07 (TMS), 2.55 (TMS).
Anal. Calcd. for
C1 9H4 8 N4 Si 4 MoO: C, 40.98; H, 8.69; N, 10.06. Found: C, 40.69; H, 8.70; N, 10.07.
[N 3 N]Mo(CNtBu) (4). [N3 N]Mo(N 2 ) (150 mg, 0.31 mmol) was dissolved in 5 mL
of toluene and cooled to -20 OC. tBuNC (39 mg, 0.47 mmol) was added to the solution which was
allowed to stir overnight. The solvent was removed in vacuo to give an orange residue. The
product was obtained as rust needles by crystallization from diethyl ether; yield 160 mg (96%). 1H
NMR(C 6 D6 ) 8 13.37 (CH 2 ), 3.76 (tBu), 0.12 (TMS), -39.00 (CH2 ). IR(Nujol, cm-1 ) 1838 (br,
C-N). I = 1.7 4 GBg.
{[N 3 N]Mo(CNtBu)}OTf (5).
[N 3 N]Mo(CNtBu) (110 mg, 0.20 mmol) was
dissolved in 4 mL of THF and cooled to -20 *C. FcOTf (68 mg, 0.20 mmol) was added to the
solution and the reaction was stirred for 25 min. The solvent was removed in vacuo and the
residue was washed with 20 mL of pentane to remove ferrocene. The residue was dried to give the
product as a burnt orange powder; yield 129 mg (92%).
(tBu), -29.38 (CH 2 ), -98.14 (CH 2 ).
19 F
1H
NMR(THF-d 8 ) 6 12.83 (TMS), 9.28
NMR(THF-d 8 ) 8 -78.64 (CF 3 SO 3 ). IR(THF, cm - 1)
2147 (C-N). g. = 2.71 ig. Efforts to crystallize 5 were unsuccessful and so elemental analyses
were not attempted.
[N3 N]Mo(CN) (6). [N3 N]Mo(CNtBu) (65 mg, 0.12 mmol) was dissolved in 6 mL of
toluene and sealed in a bomb. The solution was heated to 86 'C for 36 h during which time the
color changed from orange to yellow. The solvent was removed and the yellow solid was washed
References begin on page 137
135
Chapter3
with cold pentane; yield 51 mg (88%).
1H
NMR(C 6 D 6 ) 8 7.73 (TMS), -25.7 (CH 2 ), -112.4
(CH 2 ). g = 2.73 GBg. Anal. Calcd. for C16H 39 N5 Si 3 Mo: C, 39.89; H, 8.16; N, 14.54. Found:
C, 39.72; H, 8.31; N, 14.55.
[N3N]MoCNAr (7). [N3 N]Mo(N2) (75 mg, 0.16 mmol) was dissolved in 4 mL of
toluene and cooled to -20 'C. 2,6-Me 2 C6H3NC (25 mg, 0.19 mmol) was dissolved in 1 mL of
toluene and added to the stirred solution of [N3N]Mo(N 2 ). After 1 h the toluene was removed in
vacuo and the residue extracted with 7 mL of hexamethyldisiloxane. After filtering, the solution
was cooled to -20 'C to afford the product as red plates; yield 56 mg (62%, not optimized).
1H
NMR(C 6 D 6 ) 8 61.90 (NCH 2 CH 2 N, AVl/ 2 = 509 Hz), 37.15 (ArH, AVl/ 2 = 138 Hz), 1.87
(TMS, AV1 /2 = 47 Hz), -0.66 (CH 3 , Av 1/2 = 156 Hz)), -29.81 (NCH 2 CH 2 N, AV1/2 = 276 Hz),
-68.85 (ArH, AVl/ 2 = 882 Hz). IR(Nujol, cm- 1) 1740 (C-N). Anal. Calcd. for C 24 H4 8 N5 Si 3 Mo:
C, 49.12; H, 8.24; N, 11.93. Found: C, 48.93; H, 8.31; N, 11.82.
[bitN3N]Mo (8). [N3 N]MoCl (500 mg, 1.02 mmol) was dissolved in 13 mL THF and
placed in a bomb. Magnesium powder (30 mg, 1.23 mmol) was added and the bomb was sealed.
The vessel was subjected to three freeze-pump-thaw cycles to remove any dinitrogen present and
the solution was stirred under vacuum. After 7 days the solvent was removed in vacuo and the
residue extracted with 15 mL of pentane and filtered to give a blood-red solution. The pentane was
removed to give the crude product as a red solid (420 mg) that was (according to its 1H NMR
spectrum) a 1:4 mixture of [N3 N]MoH and [bitN 3 N]Mo. The crude yield of [bitN 3N]Mo (in the
mixture) therefore is 72%. Pure [bitN 3 N]Mo was obtained by recrystallization of the crude
product from hexamethyldisiloxane.
1H
NMR(C 6 D6 ) 8 18.58 (CH 2 ), 14.81 (TMS), 1.10
(SiMe 2 ), -18.95 (CH 2 ), -20.26 (CH 2 ), -98.65 (CH 2 ), -103.55 (CH 2 ), -125.06 (CH 2 ). g. = 2.87
GiB. Anal. Calcd. for C 15 H 38 N4 Si 3 Mo: C, 39.62; H, 8.42; N, 12.32. Found: C, 39.74; H, 8.73;
N, 12.39.
[dl-N 3 N]MoD (9). [bitN 3 N]Mo (95 mg, 0.21 mmol) was dissolved in 5 mL THF and
placed in a glass bomb with a stirring bar. The vessel was sealed and subjected to two freezepump-thaw cycles. D2 (1 atm) was introduced and the solution stirred for 2 days during which
References begin on page 137
136
Chapter3
time the color of the solution changed from blood-red to yellow. The solvent was removed and the
solid recrystallized from pentane as yellow needles; the yield was quantitative:
2D
NMR (C6 H6 ) 6
16.48 (Si(CH 3 ) 2 CH 2 D, Avl/ 2 = 6 Hz). g = 2.83 9B.
Reaction of [bitN3 N]Mo with tBuNC to give 10.
[bitN 3 N]Mo (96 mg, 0.21
mmol) was dissolved in 3 mL toluene and cooled to -20 *C. tBuNC (36 RL, 0.32 mmol) was
added to the stirred solution of [bitN 3N]Mo. After 2.5 h the toluene was removed in vacuo to give
an orange oil. The oil was extracted with 2 mL of hexamethyldisiloxane, filtered and cooled to -20
°C to afford the product as orange crystals; yield 75 mg (58%, not optimized).
1H
NMR(C 6 D6 ) 6
4.21 (t, 2H, NCH 2 CH 2 N), 3.55-3.30 (m, 4H, NCH 2 CH 2 N), 2.73 (s, 2H, CH 2 ), 2.25 (t, 2H,
NCH 2 CH 2 N), 2.21-2.01 (m, 4H, NCH 2 CH 2 N), 1.50 (s, 9H, tBu), 1.45 (s, 9H, tBu), 0.50 (s,
18H, TMS), 0.45 (s, 6H, Si(CH 3 )2 ).
13 C
NMR(C 6 D6 ) 6 247.35 (s, MoCNC(CH 3 )3 ), 167.21
(s, Mo(C(NtBu)C(NtBu)CH 2 ), 58.71 (s, MoCNC(CH 3 )3 ), 53.40 (t, NCH 2 CH 2 N), 53.35 (t,
NCH 2 CH 2 N), 52.80 (s, MoCNC(CH 3 )3 ), 52.54 (t, NCH 2 CH 2 N), 51.43 (t, NCH 2 CH 2 N),
33.02 (q, C(CH3 )3 ), 32.34 (q, C(CH 3 )3 ), 29.50 (t, MoCH 2 Si), 5.01 (q, Si(CH 3 )3 ), 3.08 (q,
Si(CH 3 )2 ). IR(Nujol, cm- 1) 1586 (C-N). Anal. Calcd. for C2 5H 56N 6 Si 3 Mo: C, 48.36; H, 9.09;
N, 13.53. Found: C, 48.18; H, 9.52; N, 13.36.
REFERENCES
(1) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9.
(2) Cummins, C. C.; Lee, J.; Schrock, R. R.; Davis, W. M. Angew. Chem., Int. Ed. Engl.
1992, 31, 1501.
(3) Cummins, C. C.; Schrock, R. R.; Davis, W. M. Angew. Chem. 1993, 115, 758.
(4) M6sch-Zanetti, N. C.; Schrock, R. R.; Davis, W. M.; Wanninger, K.; Seidel, S. W.;
O'Donoghue, M. B. J. Am. Chem. Soc. 1997, 119, 11037.
(5)
Schrock, R. R.; Shih, K. -Y.; Dobbs, D.; Davis, W. M. J. Am. Chem. Soc. 1995, 117,
6609.
(6) Dobbs, D. A.; Schrock, R. R.; Davis, W. M. Inorg. Chem. Acta. 1997, 263, 171.
137
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(7) Kol, M.; Schrock, R. R.; Kempe, R.; Davis, W. M. J. Am. Chem. Soc. 1994, 116, 4382.
(8)
Schrock, R. R.; Seidel, S. W.; M6sch-Zanetti, N. C.; Shih, K. -Y.; O'Donoghue, M. B.;
Davis, W. M.; Reiff, W. M. J. Am. Chem. Soc. 1997, 119, 11876.
(9) Freundlich, J.; Schrock, R. R.; Cummins, C. C.; Davis, W. M. J. Am. Chem. Soc. 1994,
116, 6476.
(10) Freundlich, J. S.; Schrock, R. R.; Davis, W. M. Organometallics1996, 15, 2777.
(11) Seidel, S. W., Ph.D. Thesis, MIT, 1998.
(12) Neuner, B.; Schrock, R. R. Organometallics1996, 15, 5.
(13) O'Connor, C. J. Prog. Inorg. Chem. 1982, 29, 203.
(14) K. -Y. Shih, unpublished observations.
(15) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principlesand Applications of
OrganotransitionMetal Chemistry; 2nd ed.; University Science Books: Mill Valley, CA, 1987.
(16) D. A. Dobbs, unpublished observations.
(17) Protasiewicz, J. D.; Lippard, S. J. J. Am. Chem. Soc. 1991, 113, 6564.
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Vrtis, R. N.; Liu, S.; Rao, C. P.; Bott, S. G.; Lippard, S. J. Organometallics1991, 10,
275.
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13, 1300.
(20) Giandomenico, C. M.; Hanau, L. H.; Lippard, S. J. Organometallics1982, 1, 142.
(21) Dewan, J. C.; Giandomenico, C. M.; Lippard, S. J. Inorg. Chem. 1981, 20, 4069.
(22) Adachi, T.; Nobuyoshi, S.; Ueda, T.; Kaminaka, M.; Yoshida, T. J. Chem. Soc., Chem.
Commun. 1989, 1320.
(23) Duan, Z.; Verkade, J. G. Inorg. Chem. 1995, 34, 1576.
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(28) Cummins, C. C.; Schrock, R. R.; Davis, W. M. Organometallics1992, 11, 1452.
(29) Planalp, R. P.; Andersen, R. A. Organometallics1983, 2, 1675.
(30) Berno, P.; Minhas, R.; Hao, S.; Gambarotta, S. Organometallics1994, 13, 1052.
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129, 1401.
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(35) Durfee, L. D.; Rothwell, I. P. Chem. Rev. 1988, 88, 1059.
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(38) Carnahan, E. M.; Protasiewicz, J. D.; Lippard, S. J. Acc. Chem. Res. 1993, 26, 90.
(39) Berg, F. J.; Petersen, J. L. Organometallics1989, 8, 2461.
(40) Berg, F. J.; Petersen, J. L. Organometallics1993, 12, 3890.
(41) Shih, K. -Y.; Schrock, R. R.; Kempe, R. J. Am. Chem. Soc. 1994, 116, 8804.
(42) Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.; Kim, E.; Cummins, C. C.;
George, G. N.; Pickering, I. J. J. Am. Chem. Soc. 1996, 118, 8623.
(43)
Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J.
Organometallics1996, 15, 1518.
(44) Schrock, R. R.; Sturgeoff, L. G.; Sharp, P. R. Inorg. Chem. 1983, 22, 2801.
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139
CHAPTER 4
Living ROMP of Norbornadienes
Employing Tungsten Oxo Alkylidene Complexes
A portion of the material covered in this chapter has appeared in print:
O'Donoghue, M. B., Schrock, R. R., LaPointe, A. M., Davis, W. M. Organometallics
1996, 15, 1334.
Chapter4
INTRODUCTION
The ring-opening metathesis polymerization (ROMP) of strained cyclic olefins is an
important application of the olefin metathesis reaction.1, 2 Previous work in our group 3 -5 and
others6 has shown that well-defined molybdenum imido alkylidene complexes of the general type
Mo(CHR)(NAr)(OR')2 (Ar = 2,6-iPr2C6H 3 ; R = CMe2Ph, tBu; OR' = OtBu, OC(CF 3 )2 CH 3 ,
OC(CF 3 )3 ) are effective catalysts for the ROMP of norbornadienes.
extensive studies include observations that Mo(CHtBu)(NAr)(OtBu)
2
Key findings of these
will effect the polymerization
of 2,3-bis(trifluoromethyl)norbornadiene (NBDF6) yielding all trans, highly tactic polymers
whereas employment of Mo(CHCMe 2 Ph)(NAr)[OCCH3(CF 3 )2 12 yields all cis polymers with a
bias toward one tacticity. 3' 6 Furthermore, the cis/trans content of polymers produced employing
Mo(CHCMe 2 Ph)(N-2-tBuC 6 H4 )(BiphenoBu 4 ) (BiphenoBu 4 = 2,2'-[4,4',6,6'-tBu 4 (C 6 H2 ) 2 0 2 )
was found to be highly temperature dependent, with the cis content increasing with decreasing
temperature. 7 Studies on the interconversion of syn and anti rotamers in these systems and their
relative rates of reactivity with norbornadienes led to the proposal that syn propagating alkylidene
species give rise to cis double bonds in the polymer whereas anti propagating species yield trans
double bonds. 8 The tacticity in these systems does not appear to be linked to the formation of cis
or trans double bonds but is controlled by the chirality of the 13 carbon in the growing polymer in a
process known as chain end control. If sequential monomer units add to the same CNO face of the
catalyst then an isotactic polymer results whereas if sequential monomer units approach alternate
CNO faces then a syndiotactic polymer results. In an elegant study utilizing enantiomerically pure
monomers,
5
it
has
been
shown
that
cis
polymers
produced
employing
Mo(CHCMe 2 Ph)(NAr)[OC(CF3)3]2 are isotactic whereas trans polymers produced employing
Mo(CHCMe 2 Ph)(NAr)(OtBu) 2 are syndiotactic. Chiral molybdenum imido alkylidene complexes
have
also
been
synthesized 9 , 1 0
and
complexes
such
as
Mo(CHCMe 2 Ph)(NAr') [()BINO(SiMe 2 Ph) 2 19 (Ar' = N-2,6-Me 2 C 6 H 3 ) polymerize
norbornadienes presumably via enantiomorphic site control to give polymers that are all cis and
isotactic.
References begin on page 174
141
Chapter4
In the development of new ROMP catalysts, symmetrically-substituted norbornadienes are
particularly useful as probe monomers for several reasons. First, a wide variety may be readily
prepared via a Diels-Alder reaction of cyclopentadiene with substituted acetylenes. Second, the
substituted double bond is not attacked for steric reasons. Third, the symmetric substitution avoids
head/tail, head/head and tail/tail regiochemistries thereby simplifying characterization of the
polymer by
13 C
NMR spectroscopy. Therefore, the four most likely regular structures of 2,3-
disubstituted norbornadienes are as shown in Figure 4.1.
Figure 4.1. The four most likely regular structures of 2,3-disubstituted norbornadienes.
x1's
x3 2
X
cis, isotactic
6
5
x
x
cis, syndiotactic
trans, syndiotactic
X
X
trans, isotactic
Although imido alkylidene complexes are efficient ROMP catalysts, a study of related oxo
alkylidene complexes is warranted since there is a good possibility that many classical olefin
References begin on page 174
142
Chapter4
metathesis catalysts 11 are oxo alkylidene complexes (e.g. M(CHR)(O)X 2 ; X = Cl, OR, etc.). If
oxo ligands are not initially present in these systems, they could be formed readily from traces of
water, and at low catalyst concentrations bimolecular decomposition of oxo alkylidene complexes
could be slow relative to metathesis activity.
Support for this suggestion comes from the
observation that WC16 in combination with various alkyl metal complexes such as Zn(CH 3 )2 is
inactive as an olefin metathesis catalyst when air and water are rigorously excluded but active in the
presence of trace amounts of air and water.12 Oxo alkylidene complexes have also been implicated
in the ring-closing metathesis of nonconjugated dienes. 13
Oxo alkylidene complexes are expected to be more reactive toward norbornadienes than the
analogous imido alkylidene complexes as a consequence of the smaller size and more
electronegative nature of the oxo ligand compared to imido ligands. These complexes are also
expected to be less prone to tautomerization to give hydroxo alkylidyne complexes. However, a
potential drawback of the smaller size of the oxo ligand is that the resulting complexes may be
more susceptible to decomposition via bimolecular pathways.
In contrast to imido alkylidene complexes, stable, well-defined, metathetically active
tungsten oxo alkylidene complexes are rare.
Oxo alkylidene complexes of the type
W(CHtBu)(O)(PR 3 )2 C12 and W(CHtBu)(O)(PR 3 )Cl 2 actually were the first well-defined Group 6
alkylidene complexes to be prepared but their metathesis activity was found to be short-lived, and
complexes such as "W(CHtBu)(O)(OtBu)2" were unstable. 14 - 17 Tungsten oxo vinyl alkylidene
complexes have been reported but their behavior as ROMP catalysts has not been described in
detail. 18 '1 9 Air stable tungsten oxo alkylidene complexes incorporating a trispyrazolylborate ligand
are known but are metathetically active only in the presence of a cocatalyst such as A1C1 3. 20 The
preparation of a new family of tungsten oxo alkylidene complexes is reported in this chapter.
Stable, metathetically active five coordinate complexes are accessed by replacement of the halide
ligands of precursor complexes of the general type W(CHtBu)(O)(PR 3 )2 X2 with bulky aryloxide
ligands. These complexes are potent catalysts for the living ROMP of norbornadienes and the
resulting polymers are highly cis and isotactic.
References begin on page 174
143
Chapter4
RESULTS
Synthesis of Tungsten Oxo Alkylidene Dihalide Phosphine Complexes
WO(CHtBu)C12 (PR 3 )2 complexes (P = PMe3 , PEt3 ) can be synthesized in high yield (7183%) by reaction of Ta(CHtBu)C13 (PR 3 )2 with WO(OtBu) 4 in pentane. 17 The tantalum sideproduct, CITa(OtBu) 4 , is more soluble in pentane than the tungsten species and this allows for
easy separation. The syntheses of analogous diphenylmethylphosphine complexes proceed in
moderate yields (50-60%) and the products are a mixture of the mono- and bisphosphine
complexes as indicated by the presence of two alkylidene signals, a doublet and a triplet, in 1 H
NMR spectra (equation 1). The ratio of mono:bis phosphine complex varies from batch to batch
but generally is 1:4. The complexes are yellow powders and can be used without further
WO(OtBu) 4 + Ta(CHt Bu)X 3(PPh 2Me) 2
Et 2 0/C 5
-30 oC
WO(CHtBuX 2 (PPh2Me)
+
XTa(OtBu)4
X = Cl (1), Br (2)
y=1 or 2
(1)
purification. Analogous neophylidene complexes such as WO(CHCMe 2Ph)Br 2 (PPh2 Me)y (3) are
synthesized by employing the appropriate tantalum neophylidene precursor and are isolated in
moderate yields. A second product of the reaction that yields 3 can be isolated by refrigeration of
the mother liquor. A new alkylidene species (4) crystallizes along with BrTa(OtBu) 4 and washing
the mixture with pentane yields 4 cleanly (according to 1 H NMR spectroscopy). The alkylidene
functionality of 4 is characterized by a resonance at 11.14 ppm in the 1H NMR spectrum (2 JHW =
12 Hz) and by a resonance at 295.2 ppm in the
13 C
NMR spectrum. 4 does not contain a
phosphine ligand (according to 3 1P NMR spectroscopy) and a W-O stretch could not be assigned
in the IR spectrum. Single crystals of 4 were grown from pentane at -20 OC and an X-ray
crystallographic study was carried out to determine the molecular structure of 4. Crystallographic
References begin on page 174
144
Chapter4
data, collection parameters and refinement parameters for 4 are given in Table 4.1 while selected
bond lengths and bond angles are given in Table 4.2. The molecular structure of 4 along with the
atom-labeling scheme is shown in Figure 4.2. 4 is a tungsten alkylidene dihalide bisalkoxide
complex that is related to a family of tungsten neopentylidene complexes synthesized by
Osborn. 2 1' 22 On the basis of 1 H,
13 C
and IR data, Osborn originally proposed a trigonal
bipyramidal structure for the neopentylidene complexes, 21 but the coordination environment of the
tungsten atom in 4 is clearly closer to square pyramidal with C(4) lying at the apex. The O-WC(4) and Br-W-C(4) bond angles are all close to 1020 and the O(2)-W-O(1) and Br(3)-W-Br(2)
angles open to 1540 and 1570, respectively. The O-W-Br angles are equivalent at -870 and the WC(4) bond length is similar to that found in other tungsten alkylidene complexes (see 6a below). 4
is stable at -20 °C in the solid state for extended periods of time but decomposes over the course of
several hours in solution.
In the presence of GaBr 3 or AlBr 3 , complexes such as
W(CHtBu)(OCH 2 tBu) 2Br 2 are active metathesis catalysts, 2 3 ,24 but studies of the metathesis
activity of 4 were not embarked upon, in part due to the low yield (<10%).
Synthesis of Five Coordinate Tungsten Oxo Alkylidene Complexes
As a general synthetic approach to tungsten oxo alkylidene complexes, potassium salts of
alkoxides and aryloxides are reacted with a variety of tungsten oxo alkylidene dihalide
bisphosphine complexes. This type of salt elimination reaction has been extensively utilized in the
synthesis of tungsten and molybdenum imido alkylidene complexes. 1' 25 By employing bulky
aryloxides such as 2,6-diphenylphenoxide it was hoped that isolable four coordinate complexes
would be formed. However, as will be discussed, all isolated tungsten oxo alkylidene complexes
are five coordinate in which one phosphine ligand remains bound to the metal center. NMR data
for these complexes is summarized in Table 4.3.
References begin on page 174
145
Chapter4
Table 4.1. Crystallographic data, collection parameters and refinement parameters for
W(CHCMe 2 Ph)Br 2 (OtBu) 2 (4) and WO(CHtBu)(O-2,6-Ph 2 C6 H3 )2 (PPh2 Me) (6a).
6a
Empirical Formula
C18H 3oBr 2 02W
C54 H49 0 3 PW
Formula Weight
622.09
960.80
Diffractometer
Siemens SMART/CCD
Enraf-Nonius CAD-4
Crystal Dimensions (mm)
0.33 x 0.22 x 0.18
0.38 x 0.26 x 0.24
Crystal System
Triclinic
Monoclinic
Space Group
Pi
P21/n
a (A)
8.3567(7)
12.027(2)
b (A)
10.8744(9)
19.446(3)
c (A)
12.5453(11)
19.442(3)
99.6130(10)
90
94.3660(10)
100.12
99.0200(10)
90
V (A3), Z
1104.0(2), 2
4486(2), 4
Deale (Mg/m3)
1.871
1.432
Fooo
596
1944
Temperature (K)
188(2)
187
Scan Type
o
0c-20
Reflections collected
4439
6392
Independent Reflections
3104
6055
No. Variables
209
532
R
0.0362
0.041
Rw
0.0378
0.035
GoF
1.110
1.41
a (0)
Y(o)
References begin on page 174
146
Chapter4
Table 4.2. Selected bond lengths and bond angles for 4.
Bond Lengths (A)
W-C(4)
1.868(8)
W-O(1)
1.820(5)
W-O(2)
1.812(5)
W-Br(2)
2.5613(8)
W-Br(3)
2.5503(8)
C(4)-C(44)
1.523(11)
O(1)-C(1 1)
1.438(9)
O(2)-C(21)
1.452(9)
Bond Angles (deg)
C(4)-W-O(2) 104.0(3)
W-C(4)-C(41)
139.9(5)
C(4)-W-O(1)
Br(2)-W-Br(3)
157.10(3)
O(1)-W-Br(2) 87.7(2)
O(1)-W-Br(3) 87.4(2)
C(4)-W-Br(2)
102.3(2)
C(4)-W-Br(3) 100.6(2)
O(1)-W-O(2)
O(2)-W-Br(2)
87.7(2)
O(2)-W-Br(3) 87.0(2)
101.8(3)
154.2(2)
Table 4.3. NMR data for five coordinate tungsten oxo alkylidene complexes.
Ha
8Ca
JCH (Hz)
5
10.13
287.4
119
0.35
333
6a
10.37
287.2
118
11.60
305 a
6b
10.41
284.9
121
11.40
nab
7(syn)
10.15
280.2
118
8.49
398
7(anti)
11.20
285.7
136
8.84
378
9
10.32
268.7
na
3.00
341
9.89
nac
nac
4.33
360
9.63
nac
nac
Complex
10
5P
JpW (Hz)
arecorded at -27 °C, bcoupling between P and W is not observed at room temperature, Cthermal
instability of sample prevented acquisition of 13C NMR data
References begin on page 174
147
Chapter4
Figure 4.2. A view of the structure of W(CHCMe 2 Ph)(OtBu) 2Br 2 (4).
Br(3)
C(11)
The reaction between W(CHtBu)(O)(PMe 3 )2 C12 and two equivalents of KO-2,6-Ph 2C 6 H3
yields yellow, crystalline W(CHtBu)(O)(O-2,6-Ph 2C 6 H3 )2 (PMe3 ) (5) in 76% yield (equation 2).
1H
and
13 C
NMR spectra of 5 at 23 'C exhibit sharp resonances and are indicative of only one
rotamer being present in solution with the alkylidene Ha and Ca resonances appearing at 10.13
ppm (3 JHP = 3.5, 2 JHW = 11 Hz) and 287.4 ppm (2JCp = 11 Hz, 1JCH = 119 Hz), respectively.
A JCH value of 119 Hz suggests that the alkylidene has the syn orientation, 8 as shown. Only the
monophosphine complex is observed presumably because the large steric bulk of the phenoxide
ligands prevents the coordination of a second phosphine ligand. The observations of a single
sharp resonance with coupling to tungsten in the
3 1P
NMR spectrum of 5 (0.35 ppm, 1Jpw = 333
Hz) and a doublet resonance for Ha in the 1H NMR spectrum suggest that the PMe 3 ligand is
bound to the metal on the NMR time scale. Furthermore, in the presence of -1 equivalent of PMe 3
References begin on page 174
148
Chapter4
at room temperature, resonances for both free and bound PMe3 are observed in the 1H and
3 1p
spectra, consistent with slow exchange. A strong absorbance at -960 cm- 1 in the IR spectrum of 5
is assigned to the metal-oxo stretch, characteristic of an oxo ligand that is triply bonded to
tungsten. 26 Complex 5 is stable in solution and when stored in a sealed NMR tube for a period of
years, a toluene-d8 solution of 5 remained unchanged, according to 1H NMR spectroscopy.
PMe 3
W(CHtBu)(O)(PMe 3 )2 C12
+ 2 KOAr
-2NKOr
Me
- 2 KC1, - PMe3
0
u
ArO
t
C
W
%
" I
H
OAr
5
(Ar = 2,6-Ph2 C6H3)
(2)
1 or 2 react with two equivalents of KO-2,6-Ph 2C 6 H3 in THF to give W(CHtBu)(O)(O 2,6-Ph2 C 6H 3 )2 (PPh2 Me) (6a) as a yellow, crystalline solid in 71% yield (equation 3).
PPh 2Me
W(CHtBu)(O)(PPh 2 Me)yBr
2
y= 1, 2
+ 2
2 KOAr
-2 KBr, - PPh2 Me
(Ar = 2,6-Ph 2C 6 H3 )
\ tB
ao%,
AOH
OAr
6a
(3)
In contrast to 5, all resonances in the 1H NMR spectrum of 6a are broad at 20 "C.
Portions of the variable temperature 1H NMR spectra of 6a are shown in Figure 4.3. At 20 'C the
resonance for the alkylidene proton is a broad singlet at 10.37 ppm; coupling to phosphorus is not
observed. The aryl region of the spectrum exhibits a broad, rather featureless resonance between
7.8 and 6.7 ppm. At 0 'C some sharpening of the resonances is apparent although the Ha
resonance remains a singlet suggesting that the phosphine ligand is still labile at this temperature.
At -33 °C all resonances in the spectrum are sharp, the fine structure of the aryl region is evident
References begin on page 174
149
20 oC
10.5
9.5 9.0 8.5 8.0 7.5 7.0
ppm
0 0C
11.0
11111
l l ill'i
liIl
10.34 ppm
10.0
9.0
8.0
I1I I
10.5
liIl"
7.0
"II1
ppm
I
19.5
" 1111 11
II
I II l
9.5 9.0 8.5 8.0 7.5 7.0
Figure 4.3. Variable Temperature 'H NMR Spectra of (DPPO) 2 W(O)(CHCMe 3 )(PPh 2Me) (6a).
Iii
ppm
-33 OC
Chapter4
and Ha now appears as a doublet (3 JHP = 3.5 Hz). A single broad resonance is observed at 11.6
ppm in the
3 1p
NMR spectrum of 6a at 22 'C, which sharpens upon cooling the sample to -27 'C
(1Jpw = 305 Hz). These data suggest that at or above room temperature, the PPh 2Me ligand of 6a
is labile but at low temperatures it is essentially bound to tungsten on the NMR time scale. The
broadness of the resonances assigned to the aryl protons at 20 OC might also be due to hindered
rotation of the ortho phenyl rings of the aryloxide ligand. However, the lability of PPh2Me in 6a
can be demonstrated by the addition of an excess of PPh2Me (1-2 equivalents) to toluene-d8
solutions of 6a. Variable temperature 1H NMR spectra of the relevant region are shown in Figure
4.4. At low temperatures (-33 to -20 °C), resonances for both free and bound PPh 2 Me are
observed, consistent with slow exchange on the NMR time scale. Upon warming the sample to 20
CC,
the resonances broaden and coalesce as free and bound phosphine exchange on the order of the
NMR time scale while at higher temperatures (40 to 60 °C) fast exchange occurs and a single
resonance is observed. Unfortunately, the exact rate of phosphine exchange has not been
measured. The
13 C
NMR spectrum of 6a, reveals a Ca resonance at 287.2 ppm and the 1 JCH
coupling constant of 118 Hz suggests that 6a, like 5, exists as the syn rotamer. The IR spectrum
of 6a has a strong absorbance at 957 cm - 1 which is assigned to the W-O stretch.
Crystals of 6a suitable for an X-ray crystallographic study were grown at room
temperature from a dichloromethane/pentane solution. Crystallographic data, collection parameters
and refinement parameters for 6a are given in Table 4.1 while selected bond lengths and bond
angles are given in Table 4.4. The molecular structure of 6a along with the atom-labeling scheme
are shown in Figure 4.5. 6a is a distorted trigonal bipyramid with axial and equatorial
phenoxides. The oxo ligand, Ca and Cp of the neopentylidene ligand and the oxo of the equatorial
phenoxide all lie in the equatorial plane. The phosphine ligand occupies an axial position, as
expected on the basis of the structures of five coordinate adducts of imido alkylidene complexes 27
and the structure of W(CHtBu)(O)(PEt 3 )C12. 2 8 Consistent with the NMR data, the structure is that
of the syn rotamer with the tert-butyl group pointing toward the oxo ligand.
The
References begin on page 174
151
60
-L
oC
40 oC
20 oC
00
oC
-20 OC
free PPhzMe
bound PPh2Me
-35 OC
S
I
1.6
I
I
1.5
I
I
1.4
I
1.3
I
I
i
I
1.2
I
I
I
1.1
I
l
1.0
I I
0.9
I
i
I
I I
0.8
I
I
I
ppm
Figure 4.4. Variable temperature 500 MHz 1H NMR spectra of (DPPO) 2W(O)(CHCMe 3 )(PPh2 Me) (6a)
illustrating exchange between free and bound PPh2 Me.
44.
Chapter4
W=0(1) (1.689(6) A) and W=C(1) bond lengths (1.88(1) A) and the W=C(1)-C(2) bond angle
(147.8(9)0) are similar to those of W(CHtBu)(O)(PEt 3)C12. 28 The axial and equatorial W-O-C
bond angles of 129.0 and 157.20, respectively, result in the ortho phenyl rings of the phenoxide
ligands being oriented so that they approximately encircle the neopentylidene and oxo ligands and
therefore should provide a significant degree of protection against bimolecular decomposition of
the base-free form of the complex.
Reaction of 3 with 2 equivalents of KO-2,6-Ph 2C 6 H3 in THF gives the neophylidene
complex W(CHCMe 2 Ph)(0)(O-2,6-Ph 2 C6H 3 )2 (PPh 2Me) (6b) as a yellow solid in 83% yield
(equation 4). The 1H NMR spectrum of 6b at 23 0C is illustrative of the fluxional nature of the
PPh2 Me
W(CHCMe 2 Ph)(O)(PPh 2Me)yBr
3
y = 1, 2
2
+ 2 KOAr
+ 2 KO
- 2 KBr, - PPh2 Me
(Ar = 2,6-Ph2C 6H3)
0 ",#CMe2Ph
CMePh
ArO
OAr
6b
(4)
molecule; all resonances are broadened, the resonance for the alkylidene proton appearing as a
singlet at 10.41 ppm and the resonance for the neophyl methyl groups appearing as a broad singlet
at 1.17 ppm, suggesting that the phosphine ligand is labile at this temperature. Portions of the
variable temperature 1H NMR spectra of 6b are shown in Figure 4.6. At -20 'C two sharp singlets
are observed at 1.49 and 0.92 ppm for the inequivalent neophyl methyl groups and the signal for
the alkylidene proton is a doublet ( 3 JHP = 3 Hz) consistent with the phosphine ligand being
coordinated to the metal center on the NMR time scale. As the temperature is raised, the
resonances attributed to the neophyl methyl groups broaden and then coalesce and at 40 OC a single
resonance is observed at 1.16 ppm. These data show that, as with 6a, phosphine exchange in 6b
is fast on the NMR time scale at or above room temperature. A single broad resonance is observed
References begin on page 174
153
Chapter4
Figure 4.5. A view of the structure of W(CHtBu)(O)(O-2,6-Ph 2 C6 H3 )2 (PPh2 Me) (6a).
Table 4.4. Selected bond lengths and bond angles for 6a.
Bond Lengths (A)
W-C(1)
1.88(1)
W-O(1)
1.689(6)
W-P
2.590(2)
W-O(2)
1.993(5)
W-O(3)
1.957(6)
Bond Angles (deg)
W-C(1)-C(2) 147.8(9)
W-O(2)-C(21)
129.0(5)
P-W-O(2)
167.2(2)
O(1)-W-O(3)
143.0(3)
O(3)-W-C(1)
109.2(4)
O(1)-W-O(2)
98.1(3)
O(3)-W-O(2)
87.7(2)
O(2)-W-C(1)
101.0(3)
P-W-O(3)
80.9(2)
W-O(3)-O(31)
157.2(6)
References begin on page 174
O(1)-W-C(1)
105.5(4)
154
Il.
40 oC
p__~
30 oC
10 oC
0 oC
WCHCMe 2 Ph
PPh 2Me
-20 oC
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
-0.0
ppm
Figure 4.6. Variable temperature 500 MHz 1H NMR spectra of (DPPO) 2W(O)(CHCMe 2Ph)(PPh 2 Me) (6b).
Chapter4
in the
3 1P
NMR spectrum of 6b, and the
13 C
NMR spectrum reveals a Ca resonance at 284.9 ppm
( 1JCH = 121 Hz).
W(CHtBu)(O)(PMe 3 )2C12 reacts with two equivalents of (CF 3 )2 CH 3 COK to give green
crystalline W(CHtBu)(O)(OCCH3 (CF3 )2 )2 (PMe3) (7) as a mixture of syn and anti rotamers
(equation 5). The 1H NMR spectrum of 7 has two alkylidene signals at 11.20 ppm (3 JHp = 5 Hz,
2 JHW
= 8 Hz) and 10.15 ppm (3 JHP = 3 Hz, 2 JHW = 10.7 Hz) and the rotamers are present in a
ratio of 1:9, respectively. The
3 1P
NMR spectrum of 7 has two resonances with the major one
appearing at 8.49 ppm ( 1Jpw = 398 Hz) and the minor one at 8.84 ppm ( 1Jpw = 378 Hz). The Ca
resonance for the major rotamer is located at 280.2 ppm ( 1JCH = 118 Hz) in the
13 C
NMR
spectrum and the magnitude of the coupling constant indicates that it is the syn rotamer. The Ca
resonance for the anti rotamer is found at 285.7 ppm ( 1JCH = 136 Hz). It should be noted that syn
and anti rotamers are also observed in the related vinyl alkylidene complex
W(CHCHCPh 2 )(O)(OCCH 3 (CF 3)2 )2 (PPh2Me).18
W(CHt Bu)(O)(PMe 3)2 C12 + 2 ROK
R = (CF3)2CH3C
- 2 KCI
2
-PMe
3
W(CHtBu)(O)(PMe 3)(OR) 2
7
syn and anti rotamers
(5)
Photolytic studies of 7 were undertaken to determine if interconversion of syn and anti
rotamers would occur. (Photolysis has been found to effect interconversion of rotamers in
rhenium 29 and molybdenum 8 alkylidene complexes.) Samples of 7 in toluene-d8 were photolyzed
at -45
OC
for 24 h. Photolysis was carried out at low temperature in order to retard any thermal
back reaction and to minimize the possibility of sample decomposition. After photolysis no change
in the ratio of syn to anti rotamers was observed nor was any significant decomposition evident
(according to 1H NMR spectroscopy).
This result is perhaps not surprising if phosphine
dissociation is a requirement for rotamer rotation. In 7, the metal center is rendered highly
References begin on page 174
156
Chapter4
electrophilic as a consequence of the electron-withdrawing nature of the (CF 3 )2 CH 3 CO ligand,
resulting in the PMe 3 ligand being tightly bound, even at room temperature.
Reasoning that a more bulky phenoxide such as 2,6-di-tert-butyl-4-methylphenoxide might
allow isolation of a four coordinate tungsten oxo alkylidene complex, 3 was reacted with two
equivalents of the potassium salt of 2,6-di-tert-butyl-4-methylphenol. The product, 8, is isolated
as rust-red crystals in low yield (39%). 8 does not contain a phosphine ligand (according to
NMR spectroscopy) and 1H and
an alkylidene functionality.
13 C
3 1p
NMR spectra of 8 do not contain resonances characteristic of
However, the 1H NMR spectrum of 8 does exhibit four sets of
multiplets between 2.80 and 2.18 ppm which integrate as 4 protons. On the basis of these data and
elemental analyses, 8 is formulated as the metallacycle shown in equation 6. It appears that
Me
tBu
tBu
THF
BMe
O
3 + 2 KO-2,6-tBu 2-4-MeC 6 H 2
Me
O0%
W-CH2
t
CH,
Bu
/
Me
MeMe
Me
8
(6)
replacement of the bromide ligands of 3 with the bulky aryloxide ligands results in loss of PPh2 Me
from the coordination sphere and generation of the coordinatively unsaturated species
W(CHCMe 2 Ph)(O)(OAr') 2 (Ar' = 2,6-tBu 2 -4-MeC 6 H2 ). C-H activation of the ortho tert-butyl
group then generates 8. We have seen no evidence of CH activation in an ortho phenyl ring of the
References begin on page 174
157
Chapter4
O-2,6-Ph 2 C 6 H 3 ligand related to what has been found in tungsten systems discovered by
Basset. 30
Stoichiometric
Olefin
Metathesis
Reactions
of
W(CHtB u) (0)(0 - 2,6-
Ph2 C6 H 3 )2 (PMe 3 ) (5)
Complex 5 reacts with styrene or ethylene (1-2 equivalents) in less than 1 h to yield the
corresponding benzylidene and methylidene complexes according to equations 7 and 8. The
benzylidene product, W(CHPh)(O)(O-2,6-Ph 2C 6H 3 )2 (PMe 3 ) (9) can be recrystallized from
toluene or dichloromethane/pentane to afford yellow cubes. The alkylidene Ha resonance for 9 is
found at 10.32 ppm (3JHP = 4, 2 JHW = 7 Hz) and the Ca resonance at 268.7 ppm (2 JCp = 12 Hz).
3 1P
NMR data (3.00 ppm, 1Jpw = 341 Hz) suggest that the PMe 3 ligand is bound to the metal on
the NMR time scale.
PMe 3
PMe 3
W =CO StBu
'-cH
ArO
I
Ph
+
I
toluene
H
W
ArO
H
IH
OAr
OAr
5
9
tBu
(7)
In the 1H NMR spectrum of the methylidene complex, 10, the HA and HB resonances are
found at 9.89 and 9.63 ppm as two doublets of doublets (3 JHp = 5.5 Hz, 2 JHH = 9 Hz and 3 JHP =
4.5 Hz,
2 JHH
=
9 Hz,
respectively).
A similar
pattern
is observed
for
W(CH 2 )(NAr)[OC(CF 3 ) 2 (CF 2 CF 2 CF 3 )]2 (PMe 3 )3 1 (Ar = 2,6-iPr 2C 6 H3 ) and is expected on the
basis
of
the
References begin on page 174
assumed
trigonal
bipyramidal
geometry
of
158
Chapter4
W(CH 2 )(NAr)[OC(CF 3 )2 (CF 2 CF 2 CF3)]2(PMe3) and 10. In such a geometry HA and HB are
inequivalent and hence are coupled to each other as well as to phosphorus. The
31P
NMR
spectrum of 10 exhibits a single sharp resonance at 4.33 ppm ( 1Jpw = 360 Hz), consistent with
the PMe 3 ligand being bound to the metal on the NMR time scale. Values for JCH are not
available, as 10 decomposes in solution during data acquisition. This decomposition is
accompanied by a color change from yellow to blood red and the appearance of a resonance in the
1H
NMR spectrum that is attributable to ethylene, data which is suggestive of a bimolecular
decomposition pathway. However, no products of this decomposition have been isolated.
PMe3
S\tBu
W
PMe 3
toluene
C'"
+
C2H4
O
1
=
HB(A)
HA(B)
ArO
ArOH
C"
OAr
OAr
5
10
tBu
+
-
(8)
ROMP of 2,3-Disubstituted Norbornadienes Utilizing Tungsten Oxo Alkylidene
Catalysts
To determine if complexes 5, 6a, 6b and 7 could be employed as catalysts for ROMP, a
study of their reactivity toward norbornadienes was undertaken. Both 6a and 6b react readily with
2,3-dicarbomethoxynorbornadiene (DCMNBD) in dichloromethane and the resulting polymers can
be cleaved off the metal center by addition of benzaldehyde. The polymerizations are rapid being
complete in 15 min and the polymers are isolated from the reaction mixtures as white powders by
precipitation from methanol, followed by centrifugation with yields typically being >80%.
References begin on page 174
1H
159
Chapter4
NMR spectra of the polymers, exhibit resonances at 5.42 and 3.95 ppm which are assigned to the
olefinic and allylic protons, respectively, and are consistent with a polymer that contains >95% cis
double bonds (for comparison, the allylic protons of all trans poly(DCMNBD) resonate at 3.52
ppm 3 ). The observation of a single resonance at 44.4 ppm in the
13 C
NMR spectrum that is
assigned to the allylic carbon atoms in the polymer is also indicative of a highly cis polymer. 9
Furthermore, all polymers were found to be isotactic with
13 C
NMR spectra exhibiting a single
resonance for C7 at 39.0 ppm (see Figure 4.8 for numbering scheme and peak assignments). 5 6a
and 6b also polymerize 2,3-bis(trifluoromethyl)norbornadiene (NBDF6) in toluene. The
13 C
NMR spectra of the resulting poly(NBDF6) exhibit single resonances at 44.9 and 38.5 ppm (C1 ,4
and C7 , respectively), data that are consistent with polymers that are >95% cis and >95%
isotactic. 6
Gel permeation chromatographic (GPC) analyses of poly(DCMNBD) produced employing
6a and 6b show the polymers to have polydispersities of - 1.2 (Tables 4.5 and 4.6). Furthermore,
the molecular weights of the polymers are consistently higher than expected, suggesting that kp/ki
is large although kp/ki has not been measured directly (ki = rate of initiation, kp = rate of
propagation). No significant change in the polydispersities of the polymers is observed upon
Table 4.5. GPC characterization of all cis, isotactic poly(DCMNBD) prepared using 6a.
Equiv.
Time (h)
Mn(calcd)
Mn(found)
PDI
Yield(%)
18
4
3908
7643
1.16
100
51
0.25
10779
19030
1.11
82
59
1.00
12445
20960
1.19
78 a
89
4
18691
43440
1.27b
na
aall of polymer was not weighed, blow molecular weight shoulder present.
References begin on page 174
160
Chapter4
increasing the reaction time, a result that is consistent with negligible secondary metathesis. When
the polymerization of DCMNBD employing 6b is carried out at -30 OC, the yield of polymer
decreases significantly. Assuming phosphine dissociation is required for reaction of the oxo
alkylidene complex with an olefin, the low yield might be attributable to a slowing of the rate of
polymerization due to stronger binding of the phosphine ligand at low temperatures. In related
work,7 the competitive binding of THF at low temperatures was proposed to account for the low
yield of polymers obtained when molybdenum imido alkylidene catalysts were employed. Due to
its insolubility in dichloromethane, poly(NBDF6) was not characterized by GPC analysis.
Table 4.6. GPC characterization of all cis, isotactic poly(DCMNBD) prepared using 6b.
Equiv
Time (h)
Mn(calcd)
Mn(found)
PDI
Yield(%)
95
0.25
20003
51940
1.25
87
98
0.50
20627
59530
1.18
92
108
1.00
22709
57820
1.18
93
100
2.50
21043
54910
1.30
63a
apolymerization carried out at -30 "C
As with any polymerization, a living system is highly desirable and so a more detailed
study of the metathesis activity of 5 was undertaken. 5 reacts smoothly with DCMNBD in
dichloromethane, allowing the preparation of a series of polymers of increasing molecular weight
and poly(DCMNBD) is isolated from the reactions in moderate to good yields (Table 4.7). The
relatively low yield of the 22 mer polymer is believed to arise from incomplete precipitation from
methanol rather than premature termination of the polymerization as higher molecular weight
polymers are isolated in high yield.
References begin on page 174
161
Chapter4
GPC analyses of the polymers indicate that they are essentially monodisperse (in contrast to
those obtained using 6a and 6b). A plot of Mn as a function of the number of equivalents of
monomer added reveals a linear dependence (Figure 4.7). The low polydispersities of the
polymers and the fact that their molecular weights are directly proportional to the number of
monomer units added, are consistent with 5 polymerizing DCMNBD in a living manner.
Table 4.7. GPC characterization of all cis, isotactic poly(DCMNBD) prepared using 5.
equiv
Time(h)
Mn(calcd)
Mn(found)
PDI
Yield(%)
22
4
4741
7934
1.03
56
43
4
9113
14280 (14590)
1.02 (1.01)
82
65
4
13694
19240
1.02
76
86
4
18067
27430 (27070)
1.01 (1.02)
76
129
4
27020
37530
1.03
90
166
4
34724
49050 (47160)
1.04 (1.04)
80
256
4
53463
63350 (53860)
1.05 (1.11)
90
aNumber in parentheses are duplicate runs on the same sample.
However, the molecular weights of the polymers, as determined by GPC, are consistently higher
1H
and
13 C
compatible with the polymers being >95% cis and >95% isotactic. The
13 C
NMR spectrum of a
than the theoretical molecular weights by a factor of 1.2 to 1.6.
NMR data are
representative polymer, along with the numbering scheme and peak assignments is shown in
Figure 4.8. 5 also polymerizes NBDF6 yielding polymers that are all cis and isotactic (according
to 1H and 13C NMR data).
The metathesis activity of 7 has been explored briefly and in contrast to 5, 6a and 6b, 7
reacts slowly with DCMNBD. For example, in the polymerization of 120 equivalents of
References begin on page 174
162
Chapter4
DCMNBD, after 22 h all of the monomer is not consumed (according to 1H NMR spectroscopy).
Although the polymer was not isolated, 1H and
13 C
NMR spectra of the crude reaction mixture
suggest that the polymer contains both cis and trans double bonds. These observations imply that
7 is not a practical catalyst for the ROMP of norbornadienes and further studies were not pursued.
Figure 4.7. Number average molecular weight (Mn) of poly(DCMNBD) versus equivalents of
DCMNBD added to 5 in CH 2C12.
-
y = 4712.8 + 239.75x R= 0.99421
7 104
6 104
5 10
Mn
4 104
3 104
2 104
1 104
0
0
References begin on page 174
50
100
150
200
Equivalents of monomer
250
300
163
6
7
/5
C
129
1
S4
3
2
C02CH3
H3CO2C
•
CO 2CH 3
CO
2 CH 3
C3C,
C2
C4
C5, C6
C7
160
140
120
100
80
60
40
Figure 4.8. 13C NMR spectrum of poly(DCMNBD) produced using (DPPO) 2W(O)(CHCMe 3 )(PMe 3 ) (5).
20
PPM
0
Chapter4
DISCUSSION
The major goals of this work were the preparation and characterization of well-defined
tungsten oxo alkylidene complexes and an investigation of their utility as catalysts for the ROMP of
norbornadienes. Five coordinate complexes of the general type W(O)(CHtBu)(OAr) 2 (PR 3 ) are
isolable as crystalline solids that are stable indefinitely in solution when stored under dinitrogen. It
has been shown by 1H NMR spectroscopy that large phosphines such as PPh 2 Me are labile on the
NMR time scale at room temperature whereas PMe3 appears bound to the metal center. By varying
the steric and electronic properties of the alkoxides employed, it is possible to observe syn and anti
rotamers in solution as in the case of 7.
Catalysts 5, 6a and 6b rapidly polymerize DCMNBD to give polymers that are highly cis
and isotactic. The apparent high reactivity of the oxo complexes might be expected in view of the
small size and high electronegativity of the oxo ligand. Presumably, phosphine dissociation is a
requirement for reaction of the oxo alkylidene complex with an olefin and therefore the phosphine
ligand of 5 must be labile on the polymerization time scale. In imido alkylidene catalyst systems,
cis polymers are proposed to arise from syn propagating species as a consequence of addition of
the monomer (through the exo face) to the CNO face of the four-coordinate catalyst with C7 of the
monomer extending over the arylimido ring. 8 The high cis content found in poly(DCMNBD)
prepared from 5, 6a and 6b is consistent with a similar proposal in which the oxo ligand presents
minimal steric hindrance toward approach of the monomer (equation 9).
R
R
(R = CO 2 Me)
0ihI
AreqO
I
-
H
OArax
_1_
ArO,",
ArO
CR
R
ArO
(9)
References begin on page 174
165
Chapter4
The finding that these polymers are >95% isotactic is surprising since tacticity in the oxo
alkylidene systems described here can arise solely by chain end control. In chain end control, the
tacticity of the polymer is dictated by the chirality of the f3 carbon in the growing polymer chain. If
sequential monomer units add to the same COO face of the four coordinate oxo alkylidene catalyst
then an isotactic polymer will result. One disadvantage of chain end control is that if a monomer
adds to the "wrong" COO face, then the mistake will be propagated in the polymer chain. As a
result, stereoregular polymers rarely result when achiral catalysts are employed. In fact, cis,
isotactic polymers of the type described herein, previously have been prepared only through
enantiomorphic site control using molybdenum imido alkylidene catalysts that contain chiral
chelating dialkoxide ligands,9 ,10 a process that presumably is inherently more efficient than chain
end control.
Complex 7 was found to be a poor catalyst for the ROMP of norbornadienes, a result that
suggests that the phosphine ligand in this complex is not labile on the time scale of the
polymerization. Presumably, the strongly electron-withdrawing nature of the (CF 3 )2 CH 3 CO
ligand, renders the metal center highly electrophilic, resulting in the phosphine ligand being tightly
bound. In related work, W(CHSiMe 3 )(NAr)[OCCH3(CF 3 )2 12(PMe3) 3 1 was also found to be
virtually inactive as a metathesis catalyst as a consequence of the phosphine ligand being nonlabile.
GPC analysis of poly(DCMNBD) reveals that polymers produced using 6a or 6b have
somewhat broadened PDI's of approximately 1.2 whereas employment of 5 as the catalyst yields
essentially monodisperse polymers (PDI = 1.02). In comparing the neopentylidene catalysts, 5
and 6a, the key difference is the size of the phosphine ligand which may play an important role in
the polymerization reactions.
For example, in the living ROMP of cyclobutene by
W(CHtBu)(NAr)(OtBu) 2 (Ar = 2,6-diisopropylphenyl), it has been shown that PMe 3 binds more
strongly to the propagating species than to the initiating species. 32 This difference results in the
rate of propagation being slowed compared to the rate of initiation, allowing the preparation of
monodisperse polymers.
In contrast, in the absence of PMe 3 , the polydispersity of the
polybutadiene produced is much broader (PDI > 2). Since four coordinate analogs of 5 and 6a
References begin on page 174
166
Chapter4
have not been isolated, we cannot compare the polymerization activity of tungsten oxo alkylidene
complexes in the presence and absence of phosphine ligands.
However, a comparison of
poly(DCMNBD) produced using 5 and 6a does allow us to develop a qualitative picture of the role
of the phosphine ligand as a competitive inhibitor. In both 5 and 6a we would expect that the
phosphine ligand would bind more strongly to the propagating alkylidene complex than to the
sterically bulkier initiating neopentylidene complex.
As noted in the case of
W(CHtBu)(NAr)(OtBu) 2 , such binding would slow the rate of propagation compared to the rate of
initiation. By 1H NMR spectroscopy, we have shown qualitatively that the PMe 3 ligand of 5 is
bound more tightly to the tungsten center than the PPh 2 Me ligand of 6a and it follows that the
smaller PMe3 ligand may also bind more tightly to the propagating species. This tighter binding of
PMe3 may result in kp/ki being smaller for 5 than 6a, thereby explaining the narrower
polydispersities of the polymers produced using 5. A smaller kp/ki for 5 would also explain the
observation that the molecular weights of polymers produced using 5 are closer to the theoretical
molecular weights than polymers produced using 6a as the catalyst (see Tables 4.5 and 4.7).
EXPERIMENTAL PROCEDURES
General Procedures. All experiments were performed under a nitrogen atmosphere in a
Vacuum Atmospheres drybox or by standard Schlenk techniques unless otherwise specified.
Pentane was washed with sulfuric acid/nitric acid (95/5 v/v), sodium bicarbonate, and water,
stored over calcium chloride, and distilled from sodium benzophenone ketyl under nitrogen.
Reagent grade diethyl ether and tetrahydrofuran were distilled from sodium benzophenone ketyl
under nitrogen. Toluene was distilled from sodium, and CH 2 C12 was distilled from CaH2 .
Polymerization grade solvents were stored over activated molecular sieves and a small amount
tested with a THF solution of sodium benzophenone ketyl prior to use. Benzene-d6 and toluene-d8
were pre-dried on CaH2, vacuum transferred onto sodium and benzophenone, stirred under
vacuum for two days and then vacuum transferred into small storage flasks.
References begin on page 174
167
Chapter4
NMR data were obtained at 300 MHz and 500 MHz (1 H), 75.4 MHz ( 13 C) and 121.8
MHz (3 1P) and are listed in parts per million downfield from tetramethylsilane for proton and
carbon and in parts per million downfield from 85% H3PO 4 for phosphorus. Coupling constants
are listed in Hertz. Spectra were obtained at 25 'C unless otherwise noted. IR spectra were
recorded as Nujol mulls between NaCl plates on a Perkin-Elmer 1600 FT-IR spectrometer.
Elemental analyses were performed on a Perkin-Elmer PE2400 microanalyzer in our laboratories.
GPC analyses were effected using a system equipped with two Alltech columns (Jordi-Gell
DVB mixed bed - 250 mm x 10 mm (i.d.)). The solvent was supplied to the columns at a flow rate
of 1.0 mL/min. with a Knauer HPLC pump 64. HPLC grade CH 2 C12 was continuously dried
over and distilled from CaH2. Detection was effected using a Wyatt Technology miniDawn TM light
scattering detector coupled to a Knauer differential refractometer. The differential refractive index
increment, dn/dc, is a constant for homopolymers of identical structure. The total mass method
was used to determine dn/dc which was found to be 0.096±0.005 for cis, isotactic
poly(DCMNBD) in the molecular weight range studied.
W(O)(CHtBu)C12 (PMe 3 ), 17 2,3-bis(trifluoromethyl)norbornadiene,
dicarbomethoxynorbornadiene
34
33
and 2,3-
were prepared as described in the literature. Potassium hydride
was purchased from Aldrich as a suspension in oil and was washed with pentane prior to use. 2,6
diphenylphenol was purchased from Aldrich and used as received.
W(O)(CHtBu)CI2(PPh 2 Me)y (1). y=1(20%), 2(80%). A solution of W(O)(OtBu) 4
(996 mg, 2 mmol) in 10 mL of pentane (or ether) was cooled to -30 OC. Ta(CHtBu)C13 (PPh 2Me) 2
(1534 mg, 2 mmol) was added as a solid and a yellow precipitate formed. The mixture was stirred
for 1 h and allowed to sit overnight. The product was collected by filtration, washed with pentane
and dried in vacuo; yield 701 mg (50%).
1H
NMR(CDC13 ) for the bisphosphine complex, 8
12.07 (t, 1, WCHtBu, 3 JHP = 4), 7.82, 7.68, 7.47, 7.38 (m, 20, PPh2 Me,) 2.54 (t, 6, PPh 2Me,
2 JHP
= 5), 0.69 (s, 9, CHtBu); for the monophosphine complex, 8 10.26 (d,1, WCHtBu, 3 JHP =
4), 2.40 (d, 3, PPh2Me, 2 JHP = 10), 1.07 (s, 9, WCHtBu).
References begin on page 174
3 1P
NMR(CDC1 3 ) for the
168
Chapter4
bisphosphine complex, 8 14.16 (s, 1Jpw = 331); for the monophosphine complex, 8 25.80 (s).
13 C
NMR(CDC13 ) for the bisphosphine complex, 8 323.7 (t 1, WCHtBu, 2 JHP = 10 ), 135.4 (t,
Cipso, JCP = 24), 133.7 (t, JCp = 5), 132.5 (t, JCp = 5), 131.1 (s), 130.7 (s), 130.3 (t, Cipso,
JCp = 24), 128.7 (t, JCp = 5), 128.6 (t, JCp = 5). As the product is a mixture of two compounds
elemental analyses were not attempted.
W(O)(CHtBu)Br 2 (PPh2 Me)y (2). y=1(20%), 2(80%). A solution of W(O)(OtBu) 4
(552 mg, 1.12 mmol) in 7 mL pentane (or ether) was cooled to -30 OC. Ta(CHtBu)Br 3 (PPh2 Me) 2
(1000 mg, 1.12 mmol) was added as a solid to this solution. The solution turned green in color
and then yellow. Within minutes a yellow precipitate had formed. The mixture was stirred for 9 h
and the product was collected by filtration, washed with pentane and dried in vacuo; yield 531 mg
(60%).
1H
NMR(CDC13 ) for the bisphosphine complex, 5 12.25 (t, 1, WCHtBu,
3 JHP
= 4),
7.81, 7.73, 7.46, 7.39 (m, 20, PPh 2Me), 2.71 (t, 6, PPh2 Me, 2 JHP = 5), 0.68 (s, 9, CHtBu); for
the monophosphine complex, 8 9.87 (d, 1, WCHtBu, 2 JHP = 3), 2.47 (d, 3, PPh2 Me, 2 JHP =
10), 1.07 (s, 9, WCHtBu). Due to the product mixture elemental analyses were not attempted.
W(O)(CHCMe2Ph)Br2(PPh2Me)x (3).
y=1(20%), 2(80%).
A solution of
W(O)(OtBu) 4 (1136 mg, 2.31 mmol) in 10 mL of pentane (or ether) was cooled to -30 *C.
Ta(CHCMe 2 Ph)Br 3 (PPh2Me)2 (2200 mg, 2.31 mmol) was added as a solid to this solution and
the yellow mixture was stirred for 16 h. The yellow product was collected by filtration, washed
with pentane and dried in vacuo; yield 1004 mg (51%).
1H
NMR(CDC13 ) for the bisphosphine
complex, 8 12.12 (t, 1, WCHCMe 2 Ph, 3 JHP = 4), 7.68, 7.40, 7.22, 7.03, 6.90 (m, 25, ArH),
2.40 (t, 6, PPh2Me, 2 JHP = 5), 1.14 (s, 6, CHCMe2 Ph); for the monophosphine complex, 8 9.92
(d, 1, WCHCMe2Ph, 3 JHP = 4), 2.25 (d, 3, PPh2Me, 2 JHP = 10), 1.57 (s, 6, CHCMe2 Ph). Due
to the product mixture elemental analyses were not attempted.
W(CHCMe 2 Ph)Br2(OCMe3)2 (4). Having isolated 3, the mother liquor was cooled
to -20 'C to yield yellow crystals which were washed with pentane; yield 122 mg (9%).
1H
NMR(C 6 D6 ) 8 11.14 (s, 1, WCHCMe 2 Ph, 2 JHW = 12 Hz), 7.50 (d, 2, Ho), 7.16 (t, 2, Hm),
6.98 (t, 1, Hp), 1.61 (s, 6, WCCMe2Ph), 1.40 (s, 9, OMe3), 1.38 (s, 9, OMe3).
References begin on page 174
13 C{ 1 H
}
169
Chapter4
NMR(C 6 D6 ) 8 295.2 (WCHCMe 2 Ph), 151.3 (Cipso), 129.1, 127.3, 127.0, 93.8 (OCtBu), 92.3
(OCtBu), 50.5 (CMe 2 Ph), 32.0 (CHCMe 2 Ph), 30.0 (OCtBu), 29.9 (OCtBu).
W(O)(CHtBu)(2,6-Ph 2 C 6 H 3 0) 2 (PMe 3 ) (5). WO(CHtBu)(PMe 3 )C1 2 (200 mg,
0.41 mmol) were dissolved in 5 mL THF. 2,6-Ph 2 C6 H3 0K (242 mg, 0.85 mmol) were added as
a solid and the reaction mixture was allowed to stir for 5 h. The solvent was removed in vacuo to
give a yellow film. The product was extracted into toluene and KCl removed by filtration through
a bed of Celite. The toluene was removed in vacuo to give a yellow solid. An analytical sample
was obtained by double recrystallization from dichloromethane/pentane; yield 258 mg (76%).
1H
NMR(CDC13 ) 8 10.13 (d, 1, WCHtBu, 3 JHP = 3.5, 2 JHW = 11), 7.79 - 7.65 (m), 7.61 (d), 7.37
(d), 7.30 (d), 7.26 (s), 7.24(s) 7.22 - 7.03 (m), 6.88 (t), 6.63 (d) (26, ArH), 0.77 (d, 9, PMe3,
2 JHP = 9),
0.73 (s, 9, CH-t-Bu). 3 1 P NMR(CDC13 ) 8 0.35 (s, 1Jpw = 333); 13 C NMR (CDC1 )
3
8 287.4 (WCHtBu, 1JCH = 119, 2 JCp = 11), 164.2 (Cipso), 157.7 (Cipso), 140.7, 130.5, 130.0,
127.8, 126.2, 119.9, 119.8, 43.6, 31.9, 15.52 (d, 1Jcp = 26). IR(Nujol, cm- 1 ) 966 (W=0).
Anal. Calcd for C44 H4 5WOP: C, 63.17; H, 5.42. Found C, 63.24; H, 5.60.
W(O)(CHtBu)(2,6-Ph 2 C6 H 3 0) 2 (PPh 2 Me) (6a). W(O)(CHtBu)Cl 2 (PPh Me)y
2
(393 mg, 0.56 mmol) was dissolved in 10 mL THF. 2,6-Ph2 C6 H3 0K (375 mg, 1.32 mmol) was
added as a solid and the cloudy, amber solution was stirred for 12 h. The solvent was removed in
vacuo to give an oily solid. The product was extracted into 10 mL toluene and KCI was removed
by filtration through a bed of Celite. The toluene was removed in vacuo to give a foam. Upon
addition of pentane, a yellow solid precipitated. The solid was washed with pentane until
washings were colorless and dried in vacuo; yield 383 mg (71%). An analytical sample was
obtained by recrystallization from dichloromethane/pentane.
1H
NMR(C 6 D6 ) 8 10.37 (s, 1,
WCHtBu, 2 JWH = 9), 7.60 - 6.75 (br, m, 36, ArH ), 0.87 (d, 3, PPh2 Me,
t-Bu).
3 1P
NMR(C 6 D 6 ) 8 11.6 (br, s).
13 C
2 JPH
= 7), 0.68 (s, 9,
NMR(C 6 D6 ) 8 287.2 (WCHtBu, 1 JCH = 118),
141.2, 132.9, 132.3, 130.9, 130.2, 128.6, 128.5, 127.4, 126.6, 120.3 (Ar, two peaks not
observed, Cipso are expected to be weak), 43.9 (CMe 3), 31.9 (CMe3 ), 11.2 (PPh2 Me, 1 Jcp =
References begin on page 174
170
Chapter4
25). IR(Nujol, cm- 1) 957 (W=O). Anal. Calcd. for C54 H49 0 3 PW: C, 67.51; H, 5.14. Found C,
67.78; H, 5.31.
W (
) (C H CMe
2
Ph)(2,6-Ph 2 C
6
H 30)2(PPh
2
Me)
(6b).
W(O)(CHCMe 2 Ph)Br 2 (PPh 2 Me)x (350 mg, 0.41 mmol) was dissolved in 10 mL of THF. 2,6Ph 2 C 6 H 3 0K (239 mg, 0.84 mmol) was added as a solid and the cloudy, amber solution was
stirred for 8 h. The solvent was removed in vacuo to give an oily solid. The product was
extracted into 10 mL toluene and KCI was removed by filtration through a bed of Celite. The
toluene was removed in vacuo to give a foam. Upon addition of pentane, a yellow solid
precipitated. The solid was washed with pentane until washings were colorless and dried in vacuo;
yield 350 mg (83%).
1H
NMR(C 6 D6 ) 8 10.41 (s, 1, CHCMe 2 Ph), 7.53-6.96 (br, m, ArH), 6.93
(s, ArH), 6.92-6.85 (br, m, ArH), 6.81-6.72 (br, m, ArH), 6.72-6.66 (br, m, ArH), 1.39-1.05
(br, s, 6, CHCMe2 Ph), 0.38 (d, 3, PPh2 Me, JHP = 8).
3 1P
NMR(C 6 D6 ) 5 11.4 (br, s).
13 C
NMR(tol-d8 ) 8 284.9 (CHMe 2 Ph, 1JCH = 121).
WO(CHtBu)((CF 3 )2 CH 3 CO) 2 (PMe 3 ) (7). WO(CHtBu)(PMe 3 )C12 (100 mg, 0.20
mmol) was dissolved in 5 mL of THF. (CF 3 ) 2 CH 3 COK (98 mg, 0.45 mmol) was added as a
solid. The solution changed from green/yellow to amber in color and was allowed to stir
overnight. The solution was filtered through Celite to remove KCL and the solvent removed in
vacuo.
Upon addition of pentane a brown/pink solid was filtered off. The product was
recrystallized from pentane at -20 'C as green needles; yield 55 mg (48%). 1H NMR(C 6 D6 ): Syn
rotamer 8 10.15 (d, 1H, WCHtBu, 3 JHP = 3, 1Jpw = 11), 1.92 (s, 3H, (CF3 ) 2 (CH 3 )CO), 1.76
(s, 3H, (CF3 )2 (CH3 )CO), 1.09 (s, 9H, WCHtBu), 0.95 (d, 9H, 2 JHP = 12, PMe3 ). Anti rotamer
6 11.20 (d, 1H, 3 JHP = 5, 2 JHW = 8, WCHtBu), 1.87 (s, 3H, (CF3 )2 (CH 3 )CO), 1.81 (s, 3H,
(CF3 )2 (CH 3 )CO), 1.01 ( half of doublet, other half is obscured by resonance at 0.97, d, 9H,
PMe3 ), 0.99 (s, 9H, WCHtBu).
rotamer 6 8.84 ( 1Jpw = 378).
2 Jcp
3 1P
13 C
NMR(C 6 D6 ): Syn rotamer 8 8.49 ( 1Jpw = 398). Anti
NMR(C 6 D6 ): Syn rotamer 8 280.2 (WCHtBu, 1JCH = 118,
= 90, 1 JCW = 181), 127.6, 123.8 (OC(CF3 )2 CH 3 ), 82.7, 80.9 (OC(CF3 ) 2 CH 3 , 43.9
(CMe 3 ), 32.6 (CMe3), 19.2, 17.6 (OC(CF3 )2 CH3 ), 14.9 (PMe3 , 1JCp = 30). Anti rotamer 8
References begin on page 174
171
Chapter4
285.7 (WCHtBu, 1JCH = 136, 2 Jcp = 12), 131.5, 120.0 (OC(CF 3 )2 CH 3 ), 43.1 (CMe3), 33.4
(CMe3), 19.7,
17.6 (OC(CF 3 ) 2 CH 3 ), 15.4
(PMe 3 , 1 Jcp = 30).
Anal. Calcd. for
C 16 H25 F 12 0 3 PW: C, 27.14; H, 3.56. Found C, 27.07; H, 3.70.
W(O)(CH 2 Me 2 Ph) [O-2,6-C 6 H 2 (CMe 2 CH 2 )(tBu)-4-Me][O-2,6-C 6 H 2 (tBu) 2 -4-Me] (8). W(O)(CHCMe 2 Ph)Br 2 (PPh 2Me)x (100 mg, 0.12 mmol) was dissolved in 5
mL of THF. 2,6-(tBu) 2 -4-Me-C 6 H2 OK (63 mg, 0.24 mmol) was added as a solid and the amber
solution was stirred for 16 h. Precipitated KCl was removed by filtration and the solvent was
removed in vacuo. The product was extracted into pentane and refrigerated at -30 OC to give rust
red crystals; yield 30 mg (39%).
3 JHH
1H
NMR (C6 D6 ) 5 7.57 (d, 2, Ho, 3JHH = 8), 7.22 (t, 2, Hm,
= 8), 7.15-7.05 (overlapping resonances, 5H, Hp + ArH), 2.80 (d, 1,2 JHH = 16), 2.70 (d,
1, 2 JHH = 13), 2.42 (d, 1, 2 JHH = 13), 2.21 (s, 3, C 6 H 2 -4-Me), 2.18 (d, 1, 2 JHH = 16), 2.13 (s,
3, C6H2-4-Me), 1.92 (s, 3, CH 2Me 2 Ph), 1.70 (s, 9, CMe3), 1.61 (s, 3, CH2Me2Ph), 1.41 (s, 3,
OCMe2 CH 2 ), 1.38 (s, 9, CMe3), 1.31 (s, 3, OCMe 2 CH 2 ), 1.25 (s, 9, CMe 3 ).
13 C{H}
NMR(C 6 D6 ) 8 152.3 (Cipso), 151.7 (Cipso), 142.2, 139.9, 137.7, 134.2, 131.3, 127.3, 126.9,
126.2, 125.7, 124.2, 90.73 (WCH 2 CMe2Ph), 83.1 (WCH 2 , 1Jcw = 83), 43.0, 36.7, 35.6,
35.4, 34.7, 33.5, 33.4, 32.0, 31.2, 30.5, 21.8, 21.7. Anal. Calcd. for C30 H 45 WO 3 : C, 62.33;
H, 7.58. Found: C, 62.78; H, 7.51.
WO(CHPh)(2,6-Ph 2 C 6 H 3 0)2(PMe3) (9). W(O)(CHtBu)(2,6-Ph 2C 6 H 3 0) 2 (PMe 3 )
(140 mg, 0.167 mmol) was dissolved in 5 mL of toluene. Styrene (21 gL, 0.184 mmol) was
added by syringe. The reaction mixture immediately darkened in color and was allowed to stir for
0.5 h. The toluene was reduced in volume and the product obtained by crystallization at -20 OC;
yield 93 mg (65%).
1H
NMR(CDC13 ) 8 10.32 (d, 1, WCHPh, 3 JHP = 4, 2 JHW = 11), 7.56 -
7.27(m), 7.32 - 7.27 (m), 7.23 - 7.13 (m), 7.10 - 7.01 (m), 6.97 - 6.87 (m), 6.68 - 6.61 (d) (31,
ArH), 0.47 (d, 9, PMe3
2 JHP
= 9.5).
3 1P
NMR(CDC13 ) 5 3.00 (s, 1JpW = 341).
13 C{ 1 H}
NMR(CDCL 3 ) 5 268.7 (d, WCHPh, 2 JCp = 12), 163.0, 157.5, 140.7, 140.0, 139.0, 133.5,
130.5, 130.0, 129.9, 129.3, 127.9, 126.9, 126.3, 120.3, 120.7, 14.0 (d, PMe3 , 1JCp = 26).
References begin on page 174
172
Chapter4
WO(CH 2 )(2,6-Ph2C 6 H30)2(PMe3) (10). W(O)(CHtBu)(2,6-Ph 2 C 6 H3 0) 2 (PMe 3 )
(200 mg, 0.24 mmol) was dissolved in 20 mL toluene and placed in a glass bomb. Ethylene (46.9
mmHg, 0.26 mmol) was added and the reaction mixture allowed to stir overnight. The toluene and
volatiles were removed in vacuo to give a brown/yellow solid. The solid was recrystallized from
methylene chloride/pentane as yellow cubes.
1H
NMR(CDC13 ) 8 9.89 (dd, 1, WCH2 ), 9.63 (dd,
1, WCH 2 ), 7.58 - 7.50, 7.48 - 7.34, 7.34 - 7.13, 7.11 - 6.96, (m, 26, ArH), 0.65 (d, 9, PMe3,
2 JHP
= 10). 31p NMR(CDC13 ) 8 4.33 (s, 1Jpw = 360). Thermal instability of this complex
prevented 13C NMR and elemental analyses data from being acquired.
Polymerization Reactions.
The following is a typical procedure.
A solution of
DCMNBD (60 mg, 0.29 mmol) in 1 mL dichloromethane was added to a solution of 5 (5 mg, 6.0
jimol) in 3 mL dichloromethane and the mixture stirred for 4 h. After said time benzaldehyde (30
mg, 0.28 mmol) was added to the mixture to react with any alkylidene present. The mixture was
stirred for 12 h and the polymer precipitated from methanol and dried in vacuo. Polymerizations of
2,3-bis(trifluoromethyl)norbornadiene were carried out in toluene and the polymer precipitated
from pentane. Typically, yields were >80%.
Poly - 2,3,-dicarbomethoxynorbornadiene.
1H
NMR(CDC13 ) 8 5.42 (br, m, Ha),
3.95 (br, m, Hb, cis), 3.75 (s, COCH3 ), 2.51 (br, m, He), 1.45 (br, m, Hc);
13 C{ 1 H)
NMR(CDC13 ) 8 165.4 (CO 2 CH 3 ), 142.5 (C2 ,3 ), 131.6 (C5 ,6 ), 52.3 (COCH3 ), 44.4 (C 1 ,4 ), 38.9
(C 7 ).
HeC
Ph
C4
b
MeO 2 C
References begin on page 174
\Ha
tBu
n
C1
\
2
Hb
CO2Me
173
Chapter4
Poly - 2,3-bis(trifluoromethyl)norbornadiene.
1H NMR(acetone-d 6 ) 8 5.62 (br,
13 C{ 1 H}NMR(acetone-d
m, Ha), 4.20 (br, Hb, cis), 2.82 (He), 1.56 (He).
6)
8 140.4 (C2 ,3 ),
132.1 (C5 ,6 ), 122.0 (CF 3 ), 44.9 (C1, 4 ), 38.5 (C7 ).
Ph
t
H
Hb
C3F 3C
n
C1
C4
C2
Bu
Hb
CF 3
REFERENCES
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176
Acknowledgments
IT IS DONE! The best decision I made regarding my Ph.D. work was my choice of
advisor. Working for Dick has been a tremendous experience both personally and professionally
and I think I have had the perfect graduate school experience. Thank you for always letting me
speak my mind and for allowing me to "play" in lab even when I did not know what I was doing.
I think the key to my success has been your ability to know when to push me hard and when to
gentle steer me in the right direction. Thank you.
The Chemistry Department at MIT is full of characters and the first two that I met were Kit
Cummins and Alan Davison. After I applied to MIT, Kit called me up and asked me to come and
speak with him and Alan. I guess I did something right because they gave me the opportunity to
come to MIT for which I am very grateful. Alan, my fellow Celt from across the pond, is a great
person and I have shared many a laugh with him. Along with supportive words at key moments,
he also kept me informed on the progress or lack thereof of the Irish and Welsh rugby teams. I
also wish to thank Kit whose support during my job search opened many doors for me. Steve
Lippard also spoke up for me and signed off on the Women in Chemistry Retreat which I am glad
to see is to be repeated this year.
Shortly after joining the Schrock group I crossed paths with Karen Totland, a Canadian
post-doc and soul mate. Karen took me under her wing, answered my incessant questions with
endless patience and was always willing to head out for a pint. Karen is one of the "biggest"
people I know, big in the sense of the ease with which she gives to others. The year we shared 6429 was the most enjoyable of my years at MIT and I suspect that the bond we forged will be one
of the more enduring results of my time spent at MIT. Thank you for proofreading my story and
for all the love, support and encouragement you have given me this last year. I look forward to
your visits to Geneva.
After Karen's departure I was blessed with the arrival of Yann Schrodi. Yann is another
big person and life holds much for him. I will especially miss our hugs and our biscotti breaks.
Though Yann and I have argued fiercely on occasion, we have always agreed to differ and I think
we have learned much from each other. However, I did not learn a significant amount of French
from him despite our best efforts - I will be a quiet woman in Geneva, an unnatural state for me!
Collectively, I must thank the other members of the Schrock group, past and present, who
have made the fourth floor an enjoyable place to work. The job search was a huge learning
experience in itself and I had great company in Michael Aizenberg, David Graf and Steven Reid.
Good luck on your new adventures. Thanks also to John Alexander for proofreading duties. My
final labmate is another Canadian, Jennifer Jamieson who has the uniquely Canadian habit of
playing a CD until you never want to hear it again :). Jenn has graciously dealt with my occasional
foul mouth and in these final weeks, my commandeering of the box. Thanks for your patience.
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As with most theses, mine has it's origins in the labor of others. I was fortunate to inherit
two great projects from Drs. Anne LaPointe and Nadia Zanetti who I thank wholeheartedly.
Outside of lab many a fine evening was spent with the "wine tasting" group of Deryn
Fogg, Dan Dobbs, Scott Seidel, Mike Fickes, Fred, Gretchen and Tot. I didn't learn much about
wine but I have developed quite a lip for port, the responsibility for which lies with Fickes.
Special thanks to Scotty for proofreading and for encouraging e-mails during the final months.
Gretchen also deserves huge credit for keeping the Schrock group running smoothly.
Chris Morse had the misfortune to be in the lab next door to mine and so he has had to
endure many an hour of my complaining. He has always listened graciously and somehow
managed to bolster my confidence every time. Thanks for the parties and the chocolate chip
cookies and good luck with finishing up.
Another social outlet was the First Thursday Group who provided a haven during the hell
prior to Orals. They helped put everything in perspective and kept me conscious of the fact that
there was plenty of life outside MIT. Finally, the Northeastern "Crew" were also a wonderful
source of support and will be sorely missed. Special thanks goes to Denise Messinese who
convinced me that I was good enough to apply to MIT, supported me in so many ways, and who
refused to let me quit my first semester.
Finally, I am deeply indebted to my parents and six siblings, Pod, Clare, Lynn, Tim, Dan
and Paul, who have supported all of my decisions in life. I am very proud of my family and all
that they have achieved and I love them dearly. When I graduated college in 1988, I felt a little
one-dimensional so I decided not to enter a Ph.D. program and instead I came to Boston for the
summer with a view to heading on to Australia for a year (that summer turned into a decade!). Not
all of my family understood my decision and I think they were worried that I would not return to
school. It took me five years to feel "rounded out" enough to contemplate graduate school and
MIT was the first place that came to mind. MIT was first brought into my field of vision by Pod,
the night before I left Ireland for the US and strangely enough, I think that my graduation from
MIT means more to him then it does to me. I was very fortunate to have Clare living in Boston
while I was at school and there was many a night that she and Jackie either fed me and/or gave me
a bed. Thank you. Growing up in a large family is great and it certainly knocks the corners off
you! As several years separate Paul and I from the others, we are particularly close. Paul has a
huge heart and he has been there for me in good times and in bad times. I hope that with my move
to Geneva I will get to spend more time with him. Lastly, I am indebted to my mother who has
always encouraged me to aim high and to plough my own field, so to speak. How am I doing,
Mum?
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